I pretty much did this with my Z10 350. I put the original engine on a stand and replaced it with a ZZ4. I've used aluminum valve cover adapters to attach the original valve covers and painted the block chevy orange. Too the average Joe, it looks pretty original. I was at a car show a few years ago, and a young father was pointing to my engine and told his 10 year old son that the engine was a "Rare Aluminum Headed Engine". Didn't have the heart to correct him.
Oh, the ZZ4 is a great engine, it really woke the car up, and the original is tucked safely away in the garage.