3spd to 4spd OD trans

i have been doing a lot of reading here and there. i think my 1976 Feather Duster has a A904 trans in it(3spd auto). im wanting a OD trans for when i pep up the engine so that i can still maintain a fairly high MPG. the Rear end is a 7-1/4, with what i believe are 2.76 or 2.73 rear-end gears.

im looking to find out if anyone know if and how much bigger/heaviers the A-500 trans is? if that is truely a truck trans only what would have been the sedan/coupe 4spd od trans? i have dont alot of looking on WIKI/ALLPAR, as i see 2 kinds of A5xx's and from the sounds of which both were used in trucks. if the A500 does work which one to look for the 40HR or the 42HR or was it 43HR.

thanks for the help!

*edit*
forgive me i should have looked more... i found some threads on what im after, no worries...

i guess anyone got pics of the 2 trans side by side and with the 5xx installed?