I thought this would be pretty straight forward, but it turned into a 6 month project.

Head: the head is the origiinal 1/100 Guncannon head. I cut out the visor section and used alluminum pipe to simulate the cameras. I also transplanted the MG's balljoint socket into the neck. So the head has FULL rotation.

Torso: this is what took most of my time. I kept the core block from the MG, and used the upper section of the chest from the original kit. I removed 1/4 inch from the center of the upper chest to make it more in line with the MG pieces.
I also kept the ability for the cockpit to open. To make it look like the rest of the MG line, I trimmed 4 copies of RX-78 MG chest vents together to make elongated vents for the guncannon.

Limbs: I transplanted the MG joint system into the original parts using PVC pipe and trimmed down pieces from the MG RX-78. 

This turned out to be a project and a half, but I am pleased with the results.