I need help! Info for 1972 Valiant sedan

Ok, in order:
1) Torqueflite 904
2) Many things on this site to get the power up on a 318. do a search for 318 cams or cylinder heads.
3) Swapping in a "brand X' motor/trans is WAY more trouble than modding what you have, it's a good platform.
4) Wheels are 5X4" pattern and I'll bet you have drum brakes all the way around. If you are going to be adding power you will need to upgrade to disk brakes at least in the front.
Depending on which rear end you have (probably a 7 1/4) it will need upgraded with the power increase too. 1973 up A body's had disk brakes and a more common 5X4.5" bolt pattern. More wheel choices.
5) your differential is an "open" type. to get them both to spin you need a "SureGrip" (or PosiTrac) type Differential.
All part of the rear end upgrade.