Does it just shut off like you turned off the key or does it seem to try to go into gear and stall the engine?

Just exploring the possibility of more than one problem here as unlikely as it might be. I'm thinking an internal torque converter issue as in it's locked up somehow. Engine will run in park but as soon as you pull it thru R on the way to N, R engages briefly, stalling the engine.

The no go from the key could still be a bulkhead connector problem. The one in my 66 is a little sketchy and will do the same thing.

What happens if you idle it up a bit, say 11-1200 RPM and then put it in gear?

Kevin