Adam,
The T5 works great for me, just have to remember to downshift to 4th for the decent, still running drum brakes!
This was a factory 3sd, swapped to 4 spd SM420 by original owner. It too was a 3.90:1 ratio which I destroyed in my teens.
The spicer / dana rear axle a GMC. With the tires I run (7.00R-15) I have considered the swap back to 3.90 when hauling a camping road trip load up some of the steeper grades. Most of the time I can go as fast as a 52 year old man should go. I do have a '59 261 setting in the shop that I need to get started on!
With an overdrive and perhaps taller tires you should be fine in thouse 4 strong winds with the gears you got. Keep inline my man!
BC59, Do you find that the T5 helps? I have the original 4 speed and have considered swapping it for a T5. Also, is your rear end original? Mine is, and I have 3:90 gears. I was hoping to just change out the gear sets, but I've done some looking and there doesn't really seem to be any gear sets available for my particular rear end.
