How rich are you feeling? Scoggin-Dickey has a Tremec T-56 6 speed for $6300. Richmond sells some 5 and 6 speeds that start around $1800 and should hold up fine in your application. None of them are overdriven, and I assume that is the reason you want to get the 5 speed as opposed to the 4 speed? Richmond runs lower 1-4 ratios than other manufactuers and a 1:1 5th. You could swap out your rear gear for a lower ratio and the transmission would behave like a standard overdrive transmission.

If you're feeling lucky at the junk yard, GM still puts 5 speeds in some of their trucks, they use a Getrag 5 speed in their Joe Gibbs edition Silverado and I believe a Borg-Warner 5 speed as the base transmission in the Silverado.