While you wait, you could use it with the FoV 105mm howitzer. No, it's not the intended gun to paired with this tractor, but it is in the right weight range for an M5 to handle.
Another possibility - a German 88! THere is a photo that is reproduced in a couple books of a US M4 tractor pulling an 88 FLAK. Probably pulling it to an enemy material collection point, BUT there were instances of US troops using the large amount of captured Jerry artillery against the former owners, especially when the supply crunch hit in the late fall of 1944 (due to the extended supply lines and limited off-load capability in Normandy). I'm certainly not an expert, but one might think that an M5 could handle several sizes of German guns.
Gary