First, here is what I am trying to build -- a T30 75mm howitizer motor carriage:

To do this, I am recycling a few parts from built models that were damaged during moves, plus scratching a few parts and adding some new stuff from another kit.
In brief, I am using the front cab, undercarriage and suspension from a Tamiya M21 kit (recycled).
The gun and other parts from a Tamiya 75mm pack howitizer (recycled).
Roller bumper and other parts from Tamiya M3A2 kit (new -- thanks for the bargain, Richard [Deacon]!).
Right here, it looks more like a range target than an artillery piece:

Making progress:

Note that I bored out the gun with a Dremel bit. I also fiddled with the kit sides trying to preserve some of the rivet detail, but they are pretty dinged up -- I think I will replace with sheet plastic. Next step is the fighting compartment floor and fuel cells.