m6 stuck in ?? gear will not go into neutral

Finishing up my motor rebuid we decided to tear the t56 apart. Had a slider key broke in half and the synchro for 3rd was completely gone.

Replaced those and added in a steel 3-4 shift fork.

Everything went back together pretty smoothly, but with the transmission in the car I can not turn the driveshaft with the transmission in neutral. (no the driveshaft is not connected to the rear)

Any ideas? I should be finishing the motor tomorrow or so and thought I would fire it up and ease the clutch out to see what happens.

Shifter moves fine throughout the gear pattern, and you can hear the forks/sliders moving.

so the shifter moves into the neutral position but the car doesn't seem to be in the neutral position? it could be a few things. a shift fork could be loose on the shift rail for starters...this will usually cause a sloppy shift, though. possibly a bent shift fork of some sort, but i kind of doubt it from the symptoms you named. The gears are all good, I'm assuming, so i'm not going to start there. I'd put my money on the blocking ring/synchro assembly. if you get the order mixed up, or put the blocking ring or synchro colar on backwards, the gear will not want to engage correctly. It might be trying to put it in 2 gears at once, which would lock the trans up completely. this might be your problem. a key could be out of place as well. all it takes is for one of those 3 synchro hub keys to be out of place and it will bind up the spring and hub into the collar. your best bet is to take it apart again, so nothing could possibly be damaged. get a GM manual for it. nothing is better. they withhold all the info you can ever dream of, and much more.

Well after attatching the driveshaft to the rear the tranny "popped" in to N with no trouble. Turning the rear by hand got it there. Now neutral is not a problem. But going into gears while sitting still is quite difficult.

The absolute worst is going into reverse at a dead stop. Takes alot more force/time than it did before. First is a bit of a pain, but not impossible. There is no grinding, and it seems to shift fairly normal while speeding up.

Another strange thing was if the rear is up in the air and the car is running while in neutral the rear tires will turn ever so slightly. Touching them with your finger is enough to stop them, but looks like it has a tiny stall converter.

My flywheel has been cut 2x (as far as I know) and when I used a caliper on it the thickness is .030ish shorter than factory spec. I wonder if the clutch is not releasing fully or if maybe the gm synchromesh fluid would help.

I have a manual to walk me through it, but it was actually put back together by a friend that works for a large transmission shop. (oh, they do autos though)

it will be harder to shift when you're not moving because the synchros have no effect when they are not moving. they slow gears and bring them to speed for mesh but when the'yre not moving it has little effect at all. btw, don't run the car for too long when it's in the air. it's hard on the u-joints because the operating angle increases unless yoi have a tranny jack under the rear. i'm not sure what to think about the rear wheels moving in neutral on a lift. i wouldn't worry about it.
on the flywheel, I'm not sure if having it out of spec would cause any engagement problems, but beeing cut too far past spec does have its effects. by taking that much off the flwheel, it reduces it's ability to hold shape durring heat. i'd probably try and find a new one.

it will be harder to shift when you're not moving because the synchros have no effect when they are not moving. they slow gears and bring them to speed for mesh but when the'yre not moving it has little effect at all. btw, don't run the car for too long when it's in the air. it's hard on the u-joints because the operating angle increases unless yoi have a tranny jack under the rear. i'm not sure what to think about the rear wheels moving in neutral on a lift. i wouldn't worry about it.
on the flywheel, I'm not sure if having it out of spec would cause any engagement problems, but beeing cut too far past spec does have its effects. by taking that much off the flwheel, it reduces it's ability to hold shape durring heat. i'd probably try and find a new one.

So maybe my new synchros just need to break in? Its def easier than it was the first night.

Yes I had a jack under the rear. Clutch grabs right off the floor. My specs all grabbed towards the top of the pedal, but the mcleod single disk catches and releases just above the floor.