First off, I took the time to download all SPC sets from snesmusic, testing each for the N-SPC format and sorted them out. Took several hours to do that.
Secondly, knowing the velocity/duration pointers as well as the song index in those SPC files helps me even more to convert them properly.
Last not least, I'm using a program that converts those SPCs for me to .mid and .html (logging all notes, effects, etc).
From there I convert with tinymm, insert all commands, set instruments, etc.
For anyone wanting that list of N-SPC compatible songs/games:
Code3x3 Eyes - Seima Kourinden
4 Nin Shougi
A Bug's Life
Action Pachio
Actraiser
Air Management - Ozora ni Kakeru
Albert Odyssey
Albert Odyssey 2
Alcahest
American Gladiators
Animaniacs (Super Gameboy)
Arkanoid - Doh it Again
Armor Police Metal Jack
Ashita no Joe
Asterix & Obelix
B.O.B.
BS Busters Digital Magazine
BS Dragon Quest
BS Kirby no Omocha Hako - Baseball
BS Nichibutsu Mahjong
BS Zelda
Barbarossa
Baseball Simulator 1.000
Battle Blaze
Battle Dodgeball
Battle Grand Prix
Battle Tycoon
Bazooka Blitzkrieg
Bebe's Kids
Bishin Densetsu Zoku
Bubsy in Claws Encounters of the Furred Kind
Bulls vs. Blazers
Burai - Hachigyoku no Yuushi Densetsu
Cal Ripken Jr. Baseball
Cameltry
Capcom's MVP Football
Captain Tsubasa 3
Cardmaster
Casper (Japanese)
Choukou Goukami Xardion
Clock Tower
Clock Works
Cosmo Gang the Puzzle
Darius Twin
Daze Before Christmas
Death Brade
Desert Strike
Donald Duck - Mahou no Boushi
Donkey Kong (Super Gameboy)
Doraemon
Doraemon 2
Doukyuusei 2
Dragon Ball Z - Super Butouden
Dragon Ball Z - Super Butouden 2
Dragon Ball Z RPG
Dragon Knight 4
Dragon Slayer
Dragon View
Dragon's Earth
Drakkhen
Dream Maze - Kigurumi Daibouken
Dun Quest
Earthbound
Esparks - Ijikuu kara no Raihousha
F-1 Grand Prix
F-1 Grand Prix II
F-1 Grand Prix III
F-Zero
FIFA Soccer '98 (Super Gameboy)
Fatal Fury
Fatal Fury Special (Garou Densetsu Special)
Fight da Pon!
Fire Emblem 3
Fire Striker
First Samurai
Fishing to Bassing
Football Fury
GD Leen
Galaxy Wars
Gekisou Sentai Car Rangers
Ghost Sweeper Mikami
Ginga Eiyuu Densetsu
Goods Press 3gatsu Gou
Gousou Jinrai Densetsu Musya
Gulliver Boy
Gundam F-91
Gundam Wing - Endless Duel
HAL's Hole in One Golf
Hakunetsu Pro Yakyuu - Ganba League
Hanafuda Ou
Hansei Zarujirou Kun no Daibouken
Harapeko Bakka
Harley's Humongous Adventure
Hashire Hebereke
Hebereke no Popun
Heian Huunden
Heisei Shin Onigashima
Hercules
Hiryu no Ken S - Golden Fighter
Hit the Ice
Hook
Human Grand Prix
Human Grand Prix 2
Human Grand Prix 3
Human Grand Prix 4
Humans
Hunchback of Notre Dame (Super Gameboy)
Hunt for Red October
Hyper Iria
Hyper V-Ball
Hyper Zone
Inspector Gadget
Itou Hatasu Rokudan no Shougi Dojo
Jaleco Rally Big Run - The Supreme 4WD Challenge
Jerry Boy
Joe & Mac
John Madden Football
Jutei Senki
Kablooey
Kabuki-chou Reach Mahjong Tonpuu Sen
Kawasaki Caribbean Challenge
Keiba Eight Special 2
Kid Klown in Crazy Chase
Kiki Kaikai
Kiki Kaikai 3
Kikuni Masahiko no Jantoushi Dora Ou
King Arthur's World
King of the Monsters
Kinnikuman - Dirty Challenger
Kirby Super Star
Kirby no Omochabako - Hoshi Kuzushi
Kirby's Dream Course
Kirby's Dream Land 3
Kirby's Kira Kira Kids
Knights of the Round
Konae-Chan no DokiDoki Pengin Kazoku
Lagoon
Last Fighter Twin
Legend of Zelda - A Link to the Past
Light Fantasy
Little Magic
Lord Monarch
Love Quest
Lucky Luke
Lupin the 3rd
Madden '97 (Super Gameboy)
Magic Sword
Magical Taruruto Kun
Mahjong Hishouden - Naki no Ryuu
Mario Paint
Marvelous
Metal Marines
Mickey's Ultimate Challenge
Mighty Morphin' Power Rangers
Mighty Morphin' Power Rangers - The Fighting Edition
Mighty Morphin' Power Rangers - The Movie
Motoko-chan no Wonder Kitchen
Multi Play Volleyball
Mystery Circle
Naruhodo The World
New Yatterman
Nigel Mansell's World Championship Racing
Ninja Gaiden Triology
Nintendo Power Menu Program
Nolan Ryan's Baseball
O-chan no Oekaki Logic
Oda Nobunaga - Haou no Gundan
Ohmono Black Bass Fishing
Olympic Summer Games (Super Gameboy)
Othello World
Pac-Attack
Pac-Man 2 - The New Adventures
Phalanx
Pilotwings
Pipe Dream
Pirates of Dark Water
Populous
Power Rangers Zeo - Battle Racers
Pro Mahjong Kiwame GB (Super Gameboy)
Pro Sport Hockey
Rap Jam
Road Riot 4WD
Run Saber
Ryuuko no Ken
Ryuuko no Ken 2
S.O.S.
SD Great Battle
SD Gundam Gaiden
SD Gundam Gaiden 2
SD Gundam X - Super Gachapon World
SD Hiryu no Ken
SD Kidou Senshi Gundam V
SNES Burn-in Test Cart
SNES Test Programm
Saibara Rieko no Mahjong Hourouki
Sailor Moon S - Kurukkurin
Sailor Moon Super S - Zenin Sanka!! Shuyaku Soudatsusen
Samurai Shodown
Satellaview BS-X
Secret of the Stars
Shigesato Itoi's Bass Fishing No.1
Shigetaka Kashiwagi's Top Water Bassing
Shijou Saikyou no Quiz Ou Kettei Sen Super
Shiki Eiyuu Den
Shinzui Taikyoku Igo Go Sennin
Shonen Ashibe
Shougi Saikyou
Sim City
Sim Earth
Small Soldiers (Super Gameboy)
Snoopy Concert
Sonic Blastman
Sonic Wings
Soul Blader
Sousa Sentai Wappers
Space Football
Special Tee Shot
Spirou
Star Fox
Star Fox 2
Stunt Race FX
Sugoi Hebereke
Super Adventure Island
Super Baseball 2020
Super Bases Loaded
Super Double Yakuman
Super Double Yakuman 2
Super E.D.F. - Earth Defense Force
Super Famista
Super Famista 2
Super Fire Pro Wrestling
Super Fire Pro Wrestling X
Super Formation Soccer
Super Formation Soccer '94
Super Formation Soccer '95
Super Formation Soccer 2
Super Mahjong
Super Mario All-Stars
Super Mario Kart
Super Mario World
Super Metroid
Super Oozumou Netsusen Daiichiban
Super Play Action Football
Super Professional Baseball 2
Super Scope 6
Super Shougi
Super Shougi 2
Super Slapshot
Super Soccer Champ
Super Stadium
Super Syobo Syobo CG Collection
Super Tennis
Super Tetris 2
Super Ultra Baseball 2
Super Uno
Super Wagan Land
Super Widget
Sutte Hakkun
Syvalion
TKO Super Championship Boxing
Tadaima Yusya Bosyutyu Okawari
Taekwon-Do
Tecmo Super Bowl
Tenchi Muyo
Tetris & Dr. Mario
Tetris Attack
The Mahjong Touhaiden
The Ninja Warriors
The Smurfs
Thunder Spirits
TinTin - Le Temple du Solei
TinTin au Tibet
Tom & Jerry
Top Gear
Tsuppari Oozumo Rissin Shusse Hen
Ultima 6
Ultra Baseball Jitsumeiban
Ultra Baseball Jitsumeiban 2
Ultra Baseball Jitsumeiban 3
Ultra League
Ultraman - Towards the Future
Urban Strike
Ushio & Tora
Utopia
Vegas Stakes
Volleyball Twin
Waialae Country Club
Waku Waku Ski Wonder Shoot
Whirlo
Wild Guns
World Cup '98 (Super Gameboy)
World League Soccer
X-Zone
Yadamon Wonderland Dreams
Yoshi no Kuruppon
Yoshi's Cookie
Yoshi's Islans
Ys 3 - Wanderers From Ys
Yume Maboroshi no Gotoku
Yuu Yuu Hakusho
Yuuyu no Quiz de Go! Go!
Zennihon Pro Wrestling
Zennihon Pro Wrestling 2
Zennihon Pro Wrestling Dash