I don't see any need for a single aftermarket part for the project you describe; everything you need is available in kits. Don't worry about using the AMT '68 body, it can be made to look very good. How important are chassis details to you? The '68 is workable, but finding a Revell '69 chassis is simple. They are newer tooled parts, accurately detailed, and are plentiful. It fits the AMT body with just a little work. Big block engines are everywhere: Revell Camaros, Chevelles from AMT, Revell (stock and pro-street versions), AMT '70 Monte Carlo, AMT and Revell Corvettes, and on and on...
Much depends on a specific plan.