Steve,
I don't see any problems with the conversion since you have an open driveline. Have a new driveshaft built to match your SM420, cut a new hole in the floor and you have it.
The problem that I see is that you are loosing all the benefits of the overdrive with the conversion. Providing the overdrive is working it has much better highway performance than the SM420.
Considering that the stock 55-57 donor truck either had a 3:90 or 4:11 gear set, that would be your final ratio with the SM420 as it is 1:1 in 4th gear. The OD trans is .70 above the rear end ratio which is much more engine friendly on the highway.
A later T5 might be more to your liking as they have a wide range of gears plus an overdrive.
Fred
Last edited by rfs56trk; 05/16/2015 5:24 PM.
