> I will read the Japanese text later but what model gun would they be?
They are Type 41 75mm Mountain Guns.
> Would you have any photos of the truck bed to show the mounting, or any educated guesses?
Probably, they were only placed on a platform. Sometimes, they were fixed by sandbags or wooden frames.
> But here's the photo I have which shows hardtop truck and gun plus trailer.
They are carrying searchlights. See the following page about searchlight vehicle.