If the trans is spinning around, I can assume you have slid the input shaft part-way free of the disc/pilot bearing. Otherwise, the bell-housing would still be seized on the locator pins. Am I right?
If this is the case, you are either (1) hung on your exhaust header (if you have one installed right now), (2) you are hitting the top of the transmission tunnel, or (3) you forgot to remove the shifter from the turret. We'll address those three items:
1. Remove any exhaust components near the transmission. If you have a header, get it out of there. If you have stock manifolds, make sure the down and mid-pipes are completely removed.
2. If the top of the bell-housing is hitting the tunnel, use a jack and a block of wood to very gently lift the harmonic balancer, causing the engine to tilt backwards and giving you some extra clearance. Safety first, carefulness second.
3. Don't worry, I have R&R'ed a few dozen transmissions and even I forget to do this first from time to time. Go up into the interior and remove the shifter from the turret. You have to pull the knob, then the bezel/boot, then you can get to the shifter.
If I am wrong, give more details about what you are experiencing.