I have what I believe is an original LS6 car. Everything matches the build sheet, but not the tired motor. Vin# 136370B152080. Replacing the existing 454 '78 truck motor with an original LS6 is expensive and not streetable. Should I consider a "crate 454" or rebuild the existing motor? It's a nice driver, restored and looks great, but no 100 pt. car. I want to drive it, not show it. Any advice?