Define "Budget". Neither of those engines are cheap or easy to install when compared to a gen I SBC. I would put a basic 350 together and call it a day unless you have over $5,000 to spend. Otherwise, you will get in over your head and the car will not live again for quite a long time.
Modifications for EFI include a ton of wiring, plumbing, a new gas tank (4.3 V6 Monte tank works if you use the right pump), exhaust work, possibly a transmission and a transmission computer, maybe a driveshaft, new pressure lines and hoses for the A/C and power steering, etc.