So they got you covered on the engine. But if you can build one yourself, there are several mags that detail how to build a 400 hp to 600 hp chevy 350 for around 3-4 grand. Perfectly streetable.
As for paint. I have been looking. It depends on what you are willing to do. If you can prep the car yourself, then the 199 maaco special is the cheapest route. Otherwise expect to pay around 2000 for a decent paint job with decent prep work. No special paint, just what came on it originlly. It gets more expensive the better the job you want. Ask around and check out the other paint jobs a place has done, and the works in progress before you shell out any money.
Interiors can run quite a bit for any car. To get all 4 panels redone, carpet, and seats recovered, I got estimates from 1700 to 2400 dollars depending on what you want in it.
You could go to a yard and interior from another truck, but you would probably be in the same boat.
All in all if you went the cheap route, I would guess you are going to have to dump atleast 5000 into it before you are done. If you do it right, probably double that.