First gear with a lathe motor will probably do fine - grab a smaller pinion to get a little bit more. I say this cuase I towed around a 95# dozer on a lowboy at Indy with a stock areomax. (industrial style carpet) with a stock can motor, stock transmission and running an MFU unit. Biggest issue was that the MFU couldn't handle the twim 4500mAh packs I had running it (it would over current shut down).
if it's still too fast or you want to use that three speed - i suggest a gear reduction unit. The RC4WD units that bolt inline are dcent but gear mesh needs to be watched - they don't bolt on to silver cans but I'm told they do OK with aftermarket motors.
A good friend swears by the Integy 4:1 units - I am trying one for the first time myself - they bolt up nicely and you have complete control of gear mesh. My curren tbuiild has the Integy 4:1 and an Integy 55 turn. Lots of power and lots of Torque
The integy is decently compact - due to the layout it will place the motor overtop of the servos on most trucks without moving the tranmission.
Hope this helps ....