I would suggest that you wait until the engine is running before becoming concerned with the function of the shifter.
As a 'bench test', I would recommend placing the rear end of the vehicle on sturdy jackstands, allowing the rear wheels to turn freely. With the engine NOT running, have a helper shift the transmission through the gears while you observe the shift levers. If you have difficulty with the first/reverse shift, try shifting the gears while rotating the rear wheels. Keep in mind you may have to keep one rear wheel from turning while turning the other rear wheel. This insures that the rear wheel is turning the driveshaft, which will turn the transmission gears and hopefully allow first and reverse to mesh correctly. It may also he helpful to have your assistant hold down on the clutch pedal while attempting shifting as well.
If the transmission (I am assuming it is a 3 speed) was rebuilt, chances are that the internal components are a little tight and will remain so until the transmission sees some use. When the vehicle is completed and driveable, you may have to slightly 'grind the gears' to get into first or reverse until the transmission 'wears in' a little.
I hope this helps. I will monitor this post periodically, so if this procedure does not help, I have a few more tricks to try. Good luck!
