I'm running radials on my 54 straight axle with 15" rims. Wouldn't even consider going back to a bias ply tire. Also run front and rear swaybars and disc brakes.
The truck runs straight as an arrow, no wander and can cruise effortlessly at 70 mph all day long with the T5 (.72) and 3.42 diff.
I'm running a 255/70/15's on an 8" rim with 4" backspacing on the rear.....235/70/15 on the front with a 7" rim with 4" backspacing. I have no interference problems with the front discs or tierod ends.
If I had it to do over, I would go with 235/70/15's on 7" rims with 4" backspacing all the way around so I can do a conventional tire rotation. Hope this helps.
Dave
