Vash_the_Stampede

Skipping all the junk about the transmission slipping for a few months, I'll get right to the current state. My 68 won't catch gears 1 and 2, only reverse. I've got mechanics telling me it's the transmission and friends telling me it's the linkage or the shifter.
I checked the shifter (removed the housing for it and made sure none of the components were ground down and causing it to slip out of the desired gears), seems fine. Unfortunately I have absolutely no idea how to check the linkage. I glanced in my manual and found no topic for a "linkage" (and online searches were fruitless).
Hopefully that was enough information, if anyone could give me a hand in diagnosing it or directing me to a website that has a diagram of the linkage I'd be very appreciative.