You refer to your trans as a "2 speed Hydramatic", there is no such critter used in trucks. What you have is the aluminum Powerglide. There was a TH300 2 speed, but that was only used in Buicks, mainly the Special and Skylarks from the mid-late '60s.
If you do upgrade the trans, I'd suggest the TH700R4, you'll get an overdrive that way. Especially since you do run the highways with your truck.
