New guy with a 1973 Plymouth Valiant 4 door

Welcome. I have a yellow 73 almost identical to yours. Some will tell you big block drops right in, wrong. It can be done but do your research and then decide. My quarter bet is on a 360. A host of headaches go with the big block install. Repops of floor pans are out there from single footwell sections to entire pans. You can cut quarter patches from donor doors.
The 1st thing I would do is replace the trunk lid seal and clean the quarters inside. Use some product to stop the rust growth.
The main thing is to have fun. :)