I've been told the cylinder head volume is different but I can't cite the exact size difference. Yes they will bolt up but you may have very low or very high compression. Also the lower intake of the 75 is designed for cylinder heads that are further apart and will not fit the 72. The 72 lower intake will fit but the port size is different. If I had a spare 72 sitting around I'd throw together this exact combo and check compression for you.