Generally speaking, you want to use a valve from a vehicle with about the same weight distribution front to rear. Doesn't have to be the same weight, just the same percentage F-R. So a valve from a later pickup (assuming it is known to be in good condition) would be a good choice for an early pickup. This will likely be a combination valve, with residual valving and a brake light switch incorporated into it.
That said, the new adjustable prop' valves are pretty inexpensive and easy to adjust. If you use one, you need to add a 10lb residual valve inline to the rear drums. If your master cylinder is still under the floor put a 2lb residual valve inline to the front discs to prevent fluid from siphoning back to the master when parked on an upgrade.
Joe
PS you don't say whether you have upgraded to a dual-circuit master, which I highly recommend. If you have the old single-circuit master, you will have to use the adjustable prop'valve. jw
