If the transmission has a lot of miles on it, please check first gear carefully. First gear isn't synchronized and I've seen many these having first gears with broken teeth or teeth worn at rough and jagged angles from people forcing them when they should have been a little more patient. Also if your truck spent time in a mountainous area, first gear might have been used frequently as a downhill brake and this will eventually cause first gear to wear very oddly over time. The last one of these I repaired howled like a jet engine and would jump out of first unless you physically held the shift knob constantly. Good luck. Wade's Transmission and Gear in Dallas used to be a great source of gears and parts for these. They're still in business but I don't know about their supply of old stuff.
Jon
