There is a reason GM chose an effectively single front mount. Trucks have ladder-type frames that twist limited only by the resistance of the cross members. If you mount the engine in four places, the stress of the twisting frame will also stress the motor mounts, irrespective of the engine's torque reaction, which also introduces its own twist into the frame. This is particularly noticeable on long-wheelbase truck tractors. Watch one pull off from a stop sometime. When loaded the driver's side of the bumper will rise several inches. Same is true pulling a hill down in the gears.
There is significant twist between the cross member under the bell housing and the front one where the engine mounts. One the one-tons and up, the whole front sheet metal is mounted on one point at the center so the frame can twist under the fender/hood/radiator assembly without tearing it apart. On half tons this is not true and there are cross-braces ahead of the radiator where as on the bigger trucks the braces go to the mount point at the center. Since the half tons aren't supposed to be loaded so heavily the frames aren't supposed to twist so much. I don't know which way three-quarter tons are rigged.
If you're dealing with a half ton, you might get away with it but why? The factory system works well. The holes aren't hard to drill.
