Is there a complete list out there of differences between the SMB1 ROM files from the FDS release in Japan and the NES release in the U.S.? Normal gameplay is apparently identical, but there are clearly differences in the ROMs (e.g. in world 36). I haven't had any luck looking on Google.

Pretty sure the only different is the Minus World (like you said), where there's 3 levels on the JP version and 1 level that's different on the US version. I couldn't find any list of the differences on the internet.

But the fact that that world is different means that at least some of the ROM is different or in a different order. One video I watched claims that in the NES version, the address checked when attempting to warp to world 36 is in the enemy data address table. So presumably then, either the enemy data address table is different in the FDS version (because the enemy data is in a different place or the FDS addressing works differently) or it is in a different place in the ROM. So that's at least one difference.

I would just run a diff, but I can't tell which ROM files online come from which game, since most are labelled US/Japan.

FDS PRG data is loaded in at $6000-$DFFF instead of $8000-$FFFF, even if the actual code were as identical as it could be, that would be enough to change minus world since everything is moved into different locations.

You won't be able to diff the files since FDS is a whole different format, but the "US/Japan" rom you're looking at is likely the cartridge version. The FDS version would have an .fds extension, and not have "US" in the name.

If you want to actually compare them, you could start both versions of the game in an emulator with a decent disassembly view, copy out the disassembly and diff those, then account for the differences caused by the different load location vs actual code changes.

Thanks. I'll look into that.