When you say GM do you mean Chevy? If you want to convert to a Chevy, you need to put the Quadratrac output shaft into a Chevy TH400 transmission case. The TH400 behind your 304 has an AMC case. Unfortunately, the output shaft is the very last thing to come out of the case when you take the transmission apart, so you'll need to completely tear down the TH400 to swap to a Chevy transmission case.
1) '77 J10 LWB, 258/T15/D20/3.54 bone stock, low options (delete radio), PS, hubcaps. 2) '82 J20 360/T18/NP208/3.73, KOs, 7600 GVWR 3) '75 CJ-6, 304/T15, PS, BFG KM2s, soft top 4) '95 VW Golf Sport 2000cc 2D