I have a similar problem, and I posted about it earlier ("Concerned synchros are failing", or something like that). Right now I'm just living with double clutching, but the concensus was that it could be the clutch or tranny input shaft support bearing, or the synchros or fluid. A spline on the clutch spring could be bent, causing a small amount of pressure on the flywheel, or the bearing in the flywheel that supports the tranny input shaft could be seized or stiff, also transmitting a small amount of power to the tranny with the clutch out. You said you adjusted the clutch and changed the fluid...If it's the synchros on 1st and 2nd you're looking at rebuilding that tranny or finding a used one. Did you try the reverse test? While stopped, shift into first, then reverse, then neutral, wait several seconds, shift into reverse, repeat neutral-wait-reverse several times. Reverse should never grind, and after reverse the first time, there should be no resistance shifting, if it does, the input shaft is turning.

As a side note, my reverse never grinds, but perhaps if the bearing is only stiff, it would keep the shaft spinning, but not start it spinning from stop? I'll try selecting reverse at high rpms.

Good luck,
Richard
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'90 744 tic