The only modifications I made at the time were the hinges for the doors and engine hood as well as adding the hoops over the personnel body. I also opened the windshield (fresh air mode).
It was a relatively simple kit....proportions weren't perfect and the wheels didn't have quite the correct shape, but it was a fun build back then.
Incidently, The GMC 'deuce and a half' 6X6 trucks used a GMC motor (slightly larger external dimensions), while the shorter Chevrolet 4X4 versions used the Chevrolet motor.