I agree you need 2.5" pipe. Also I would call around the dealerships in the area for a price on the engine. They may have a sale going on and not cost in shipping. Also do a search on google for that GM part number and see what comes up. I bet Jim Pace and others can beat Jeg's.
Look here
350HO This is where I bought mine from Jason. He works at the Cadiallac dealership in Florida but sells on the internet. It's one of the lowest prices I've seen. Like I said do a search for prices to see who may have the cheapest around. Same part with warranty but different sellers.
Eric- Great idea but you thought about getting a ZZ4 shortblock minus the cam and timing gears? Then you have a great shortblock then add your cam, heads and intake of aftermarket companies and you got a hell of an engine.