The SM420 is not an OD, it is 1 to 1 in 4th gear just like your original trans. T-5's are light duty and probably would be unsuitable for a larger vehicle. It is also unlikely that you can fit a Gear Vendors OD unit in the space between the trans & transfer case. You might look for a 5 speed from a late model full size truck. You are in for modifications to several components no matter what you select.
Fred
