Put it in first, press the clutch in, start it and hold on!
I assume you have the linkage hooked up where it was before.
Also, you could have "racked" or run through the gears before installing it. I wouldn't think that it would be the transmission if it was rebuilt by a shop.
Do you feel the pressure on the clutch pedal about halfway down where it should be?
I'm leaving to go to work, good luck with it.
I feel pressure on the clutch and it will come up on it's own accord.
We did start it in gear and it took off however, it difficult to change gears, my dad could get it sometimes but then it stopped shifting. He would gas the engine and press the clutch and it still wouldn't go into gear.