I think zenish might be right;you might be able to add your overdrive rear section onto your "new" 3 speed.
If you're having trouble with the linkage,the forst thing is to disconnect it from the transmission. Put both shifting arms on the transmission in the center position. That's neutral. Move the shifter until you find the spot where you can move it from the 1st/reverse positions to the 2nd/3rd positions. Find a way to lock the shifter there. It'll be in neutral too. It's very important to have everything centered in it's neutral position.Slide under the truck and see where your linkage lines up to the shifting arms. If they're off by more than 1/8",try to adjust them to fit easily into the holes in the shifting arms.If you have room,shim all the linkage up so there's just a little slack with flat washers that fit well. A little grease between them is a good thing. You don't want them to fit really tight,but you want as much slack out of them as possible. Now (here's the fun part)have your wife slowly run it through the gears a couple of times with the engine OFF while you watch the linkage from under the truck,to be sure nothing's binding or snagging on other parts.When you're done,it should shift like a new transmission.
