I am in the process of turning my little Special into a "sleeper" if you will. Im keeping the exterior the same, other than throwing on some classic Rocket wheels. Interior will be kept the same, just added 3 late 60's gauge cups. Now engine is a different story. I just put the Buick 350 in, with NO modification to the heater box, steering or anything like that. Hell, the mounts are even stock. When you open the hood, everything will look factory.
