The Stovebolt Site Forums The Parking Lot Big Bolts brownie box with an Auto tranny?

At 15,000lb you are probably pushing the limits of what your 350 is capable of. The problem you are looking at is that even if you do add an auxiliary trans you may not get the right split and still not be able to pull the hill in the new split ratio. Your 4L80 is 1:1 in 3rd and .75:1 in 4th. You would probably need to accomplish a .85 or higher to have a chance of being able to pull in that ratio, otherwise as others have said you will simply not have enough torque at the lower engine speed.

Tad