the only difference between the 70-74 and 75-81 is the rear body bushing location the frame is the same length and the same width ( people get confused because of the trans cross member being different, that is because it is, and bolts to different holes).
What you need to do is drill the frame for 4 trans cross member holes to mach the early frame and drill 2 1/12" for the rear body bushings to mach the early frame, that's it