http://www.primeportal.net/artillery/da ... 155mm_hmc/

The Bulldog hull is rwd, the Gorilla is fwd. I couldn't just reverse the lower hull as the suspension arms would point the wrong way. I had two choices. Slice off each side and swap them over, or slice off the front and back and swap those, I opted for the second one as that was easier. If you look at the way the Gorilla ones are mounted, the rear two will still be wrong, but at least it will work on the model.
I then had to re-attach them, and make them both square and rock solid, and fill in all of the gaps.
Once I had done this I knew the length that I needed for the upper hull. This was a pain to do as the Gorilla is narrower than the HL Bulldog so I had to use blueprints as a guide, but stretch the design across the new hull dimensions.
So far it's turning out ok, still much glueing, filling and sanding to do, as well as a lot of printing and designing, but its getting there.