Building a surprise 67 Cuda for my Pops

This is Awesome! Great thing you are doing.
I would swap out the 7 1/4 rear for the 8 3/4 rear will be worth it in the long run.
73 to 76 disc brake swap I think will be the cheapest and easiest to get parts. But if you want to keep it small bolt them do the stock 4 piston disc brake. Not that bad if you rebuild them yourself. Rebuild the complete front end will make a huge different (find out what brake you are going with first) plus a sway bar. Small cam up grade with LB4B intake with 4 barrel. Others will give you better detailed info on this stuff.