What is the usual tractor for one of these, a 251?
Official approved vehicles to tow a PaK 40 included SdKfz 11, 3-ton truck (Opel Blitz, Mercedes L3000, etc), SdKfz 251, RSO, and Maultier.
From 1942 to mid-43, an SdKfz 11 or a 3-ton truck would be most common, after that an RSO or Maultier or 3-ton truck would be most common. There are many photos of PaK 40's being towed by SdKfz 10 - this is due to the fact that PaK 40 was replacing the PaK 38 in many existing units; they got new guns but no prime movers. The PaK 40 is 1/2 ton heavier than the SdKfz "official rated" towing capacity, but photographic evidence (and "use what you have" in reality) suggested otherwise. There are also photos of PaK 40's being towed by Steyr 1500, which is also a bit light of a vehicle for the job.