I started this upgrade to a '74 I'd been working on.
The thing about the Revell kit is because it's based on the die-cast, it's flared out too much at the rocker panels below the doors. It should be considerably narrower there but you'd never be able to spread the metal body out enough to get the chassis in. IIRC, I trimmed about 1/8" off both sides between the wheel wells and some from the rear wells. It's still a tight fit.
I also cut the underhood area from the Revell kit and fit it to the MPC body. I plan to use the complete interior too. Basically just swapping the MPC '74 body for the Revell '77.