Andy,
Have you tried to shift gears with the clutch in and the truck rolling? Having the engine locked up the input shaft on the transmission is not moving at all. This could be keeping the gears from lining up...there may not be anything wrong with the trans.
If you know you have stuck valves they could be locking the motor up. Take a big rubber mallot and hit the top of the rocker arms to see if the valve is stuck or not. If you can feel the spring bounce the valve if they are ok. If you find you have some stuck, loosen or remove the rocker arm assembly, this will allow the engine to turn with the valves out of the picture. This will also save your valve train from any further damage (bent push rods) as you try to turn the crank. If you remove any of the push rods remember to mark them so they can go back in the same hole later.
Happy Holidays!
Mike B
