Chuck,
One of the really major differences is that the 70 was rated at gross HP while the 76 is rated at net HP. Both ratings are on an engine dyno and taken at the flywheel. The gross is without any accessories, open air intake, headers with open exhaust, no emission equipment, no alternator, no water pump, etc. Net is as the engine is equiped installed in the car, full air cleaner assembly, stock exhaust mainfolds with stock exhaust and mufflers, alternator, water pump, and full emission equipment.
The big restriction on the 76 is the exhaust. The single exhaust (2 into 1 into 2) and a single cat converter kills the engine. Also the 70 350hp engine is more like the L82 of later years then your base L48. Liven it up with the normal hot rod tricks, exhaust, cam, heads, etc.
tom...