There was an article written by GM when they released the Lumina in 1990. After they completed what they were sure would be the design with the 3.1 V6, they said "we just had to put a 305 in it". The whole article describes how they did it. Look up Chevy Lumina 305 on google, you might find it.
Anyways it would be tons of time. If you were hell bent on a v8 in a Grand Prix the Cadillac 4.1 would be easiest because that car was front wheel drive.