You wouldn't edit the .jar to change a single level's gamemode... Anyway, I said I knew of some standalone programs which could change the player's gamemode in a world. TooManyItems gives the player too much power and temptation, as well as Single Player Commands. But the standalone programs are probably outdated by now, so...
Mr. Jelly I know you got your mods working, but if you want it a little easier in the future:
Mr. Jelly, if installing mods is such a hard time for you:

All you do is download the mods as .zip/.rar/.whatever files, probably should put them all in one folder. Then you use this launcher, find those two zips and add them to the list. If you have a bad status about Modloader, use the Up and Down buttons to move them so Modloader is first. Then, when you run the game, these run with it.