Why do you need to do a full rebuild if it's just got a dead engine? Drop in another 350 and drive the sucker. No point going back to a 253 if you're not going to sell it. Even if you did sell it the 350 would probably add a bit of value cause it would widen your target market and give the new owner some choices. It's much easier to buy one with a 350 already registered than to try and convert to 350 and then get registered