I'm converting my 1983 GSL to a GSL-SE 13B (factory fuel injection).

Any differences between an 83 fuel tank (12A) and the 84/85 fuel tank (GSL-SE)?

I understand the Series 1 (79/80) have a full size spare tire and a smaller fuel tank, and the Series 2 (81/83) and Series 3 (84/85) have a space saver spare tire and a larger fuel tank.

 

I did the swap on an 85 GS and I used a GSL-SE fuel tank. I feel as though this is your best option. You will also need to make your own fuel feed line, as the GSL-SE fuel feed line is larger that runs to the engine bay. I used a roll of aluminum fuel line that I bought from Summit Racing to make my feed line. I ran the section I needed and flared each end. I removed the original fuel return line and used the original feed line as my return. It worked perfectly.

 

An 85 tank won't fit in an '81-83.

The feed line is fine for EFI, it's 8mm like a whole lot of other things. The largest issue is that there isn't as much baffling in the carbureted tank. This can be solved with using the existing fuel pump as a feed pump to an external surge tank, which feeds the EFI pump.