The 904 evolved into the 500 which was later renamed the 42RH. The 727->518->46RH. 1996+ went to the RE models which are useless to you so make sure you don't get one of those.

Last time I checked, which was many years ago, the torque converter selection for the lockup units was poor. The non lockup units can use a 727 converter but I have read a loose converter with overdrive can have slip problems at cruise. I have read the 4sp's can be made to fit under a e body/71+ b body without cutting the crossmember with some grinding to the trans and careful fitment. I have read others who went to the GM 4 speed autos for better/cheaper aftermarket support and a smaller physical size to not have to cut. I ended up going with a stick because I don't like autos anyway.