As said, the 251/4 could not carry one of the guns noted, it was used to tow a variety of guns and carried ammo and other stuff for them. There are some versions of the 251 specifically modified to carry larger caliber guns, one of them was the 251/22 with the 75mm PaK. Others include the /9 with short 75mm gun or /10 with 37mm PaK.
Heck they even tried to install the 88mm PaK in a 251 but this failed. The 251 probably desintegrated from the recoil force.