Check the trans splines and make sure the're clean clean clean. Try your clutch disc on the splines too. I had exactly the same problem. I was using a tranny that had been sitting outside under a tarp for a few years, and the splines had got a little bit rusty. I wire brushed them, and they looked fine. After much grief I finally managed to force the engine in (bad idea, as we all know). It started but the clutch wouldn't release because it was binding on the splines. Hope all this helps.