OK, well what I said isn't entirely true. I've read that the SdKfz 251 could tow the '88 but at not fast enough to keep up with the panzers. Not really an effective way to keep the AA guns up near the front.
For the M3, I remembered reading its engine was only around 90 HP, compared to its actual 147. The squad and i were taking about the vickers 6ton tank, so I probably pulled the 90hp from that conversation. Sorry about that.