Yesterday I dove into the M41 and promply discovered that the trouble was exactly the one diagnosed by Matt and Pete. The dogs for the forward syncro were lolling around in open air. This is where the voice of experience is of great, great help: I had been looking at them the whole time without understanding that they weren't supposed to be like that. Tore down and reassembled the transmission and it shifts beautifully.

As to the tight or binding spot in the engine's rotation I guessed Ron's alignment-problem suggestion was probably right and switched back to the original bellhousing--I had used a NOS one I had around, simply because it was clean. This time around, no problems at all.

Engine and trans are back in and the gear oil's off the floor...thanks everyone for the advice; if it ever thaws outside we'll see how it runs.

Best,
N