Engine and tranny help

Congrats on having a car crazy son! Now tell him to get a paper route to help pay for it so he'll appreciate the car a whole lot more! ;)
And then get him multiple subscriptions to the various MoPar mags!

The V8 conversion is a little pricey when all is said and done. I did it once. Ya gotta want to do it. Your early year A body can use cheap motor mounts from Trans Dapt IIRC.

I suggest a 904 trans. A little cheaper and lighter in weight. It also uses less power to run.
Headers? I'm not very good with the older cars. You'll need others to chime in.
The rear end is probably a 7-1/4. They are weak. The 8-1/4 is the min. Size to use and the pricey but very strong 8-3/4 is the best to have.