Just checking...we're talking about a stock 1955 2nd series (TF) 3100 pickup...right? If so the trans just hangs on the back of the bellhousing, there is no crossmember.
If your truck has been modified we would need more info on how the current engine and trans are mounted.
Seems like a lot of work for no gain...both transmissions have a 1:1 high gear, so top speed will be exactly the same. A T-5 trans would give you OD and take about the same amount of work.
Mike B
