Will a engine from 1996 Corvette fit on the mounts in 1975 body?

No, a 1996 Corvette engine (likely a LT1 or LT4 small-block V8) will almost certainly not fit on the engine mounts of a 1975 Corvette. The chassis and engine bay are significantly different between those two generations. The 1975 car uses a much older style of small-block, and the mounting points, accessory locations, and even the overall dimensions of the engine bay are incompatible. You'd need extensive fabrication and custom work to even attempt such a swap, and it wouldn't be a simple bolt-in affair.