I personally think the 3/4 ton frame would be a good one to use. It is a little thicker in gage than the 1/2 ton. The front of the frame is the same, but the rear of the frame has parallel frame rails instead of the straight tapers front to rear of the 1/2 ton. With that in mind; mounting the bed and rear bumper will be different, but not impossible. Shortening a frame that is tapered from front to rear could prove to be problematic depending on where the taper stops and the parallel section starts.

Remember to us complete joint penetration welds when joining the frame sections and use reinforcing in the form of boxing plates or cee shaped bent plate; at least as thick as the frame material.

If you are familiar with a torch and welding machine, I would not hesitate. Remember to level and square the frame and measure 10 times before you start cutting. Also add bracing where needed to maintain the shape of the frame after it has been cut.

Something else to be mindful of is that the 3/4 ton is setup for the rear springs to be on top of the axle whereas the 1/2 in under the axle. After market or custom rear springs and mounts will probably be needed.

54 Chevy build pics.