The only difference in a 228 and a 248 engine is the size of the pistons and rings. The crankshaft, rod and main bearings, and virtually everything else in the engine is the same. Adding 20 cubic inches will increase the horsepower and torque the engine develops a little, but I'd prefer to keep the otiginal engine and rebuild it to "like new" condition. That being said, the price you've been quoted for the rebuild is just shy of highway robbery. Shop around for an experienced engine rebuilder who isn't trying to get rich by fleecing his customers. Parts for GMC's are scarce and expensive, but they're still available if you shop wisely. Ebay has a surprising number of parts for antique engines, including GMC sixes.
Jerry
