This picture should help you out. This is a Revell Z06 engine mated to the tranny from an AMT 1965 El Camino.
Start by gluing the engine halves of both motors together. Then, using a razor saw, carefully separate the engine blocks from the bellhousing on both. Finally, glue the ZR1 block to the bellhousing from the Chevelle. You will have to modify the oil pan on the ZR1 to fit the Chevelle chassis. Don't worry about accuracy, it is a model so feel free to take liberties here. Let me know if you have any other questions.