Want the pak-40, would be simple to do (take the panzer's gun mounted on a carrage, and you have a Pak-40. The Kwk 40 is just a tank mounted version of the Pak-40)
Have them be able to be hitched to the M3 and the German halftrack (never bothered memorizing its designation since I never use it, and you never see them), towed to the general area, and let them man handle their guns into position (anywhere within 300 yds or so), and say that if they move out of the 300yd circle, they only have the round in the breech, and have to be in that circle to have any more ammo.
Also let them "dig in" by putting a small dirt mound in front of them up to the point that the gun can fire level, and then make it so the gun will only move within its limited traverse, and if they want to adjust it, they have to move the gun (may not be as protected by the entrenchment).
Reasons for inclusion:
Will be great for setting up in the maps with V bases in the canyons (set up when your side is getting hammered, and have your guys retreat past them so the enemy blindly follows into a trap.
Will let the M3's have some defense by towing an AT gun with them.
Great town defense weapons since you can put them in the little alleys tanks can't fit into.
Its easy, as I said, just take the Panzer's gun and mount it on a carrage.