On a tight budget will need a lot of planning to ensure you can do what you want without blowing the real budget or ending up with a garage full of what was a car that you lost interest and money in. Give yourself a real chance to do this by making informed decisions, it's a big deal knowing what to do and a bad deal guessing as you go.
I think 78-80 monty and regal, some had aluminum hood and trunk lid options so that's something to make note of! there's a bunch of Factory upgrades you can find at junk yards and pull-a-parts like f/r away bars and front sub frame braces!
Them 8.5's are expensive! I used a newer 7.625 s-10 posi with new stronger gear set and a custom fit spacer(NOT a crush sleeve)that alone helps 7.5 big time! The 7.625 has 2 more spline and is same as gn's thicker stronger axles but needed to be shortened a hair(grinder) and c clips needed massaging to fit but it held up compared to the 3 stock 7.5's i destroyed previously. I had the ring and pinion shop install the gears and carrier because that's what they do and it's a huge deal that it's done correctly, add a aftermarket cover with built in main cap support and top quality lube. It did cost around 550 total but I was happy and it lasted unlike the factory pieces. No it's not as strong as a 8.5 but can handle a build like this with a sorted suspension(no wheel hop issues) problem free. And there are other options, take the time and find this info then make a informed decision. Time is in your hand not money!
The best thing I did was gut the car and throw out all factory soundproofing and carpet, coat the inside with spray cans of truck bed coating then install a material of your choice. I saved about 40lbs alone and interior noise was same or better. Factory front seats/bench are massively overweight, do your build a favor and get something lighter because there was a 45lbs + savings there.
The weight is a key factor and there are many many ways to lighten up without spending a lot of money.