I don't know what 40's ford pickups with the open drive line used for motor mounts , when a stick trans with open drivelines were installed in a car you could get some clutch chatter because of the engine moving foreward and backward, the stock torque tube drive shaft bolted to the back of the transmission, keeping it from moving. In the old days there was a fix . There were U shaped brackets with a bolt in the bottom of the U that you slipped over the ctock ford front motor mount rubber "biscuits". You could tighten the bolt to preload the mount. I don't know how good they worked. I don't think my son used any when he had a flat head Merc in his 50 ford pickup.