Roger,
In my opinion, the OD swap is the best thing you can do for these old trucks. The OD is .70, so with a 390:1 rear end you end up with a 273:1 final drive ratio. This will reduce your RPM's considerably. I run mine on the So Cal freeways with the crazies all the time. You also might want to think about upgrading to disc brakes though.
Fred
