Two things come to mind here, but a little clarification may help. You neglected to specify whether the trans fails to pull out of first gear on a high rpm shift or rather it only gets to a neutral position. The gears in a big mopar 4spd are bigger and heavier than a Muncie or Borg Warner T10 GM trans therefore as rpm increases it is a bigger job for the tapered cone brass synchro rings to slow down the next coming gear. GM transmissions shift like butter at high speed, but are much weaker than the big Mopar gear box. My guess would be a worn or cracked brass synchro ring on 2nd gear or grinding burrs on the 1st gear iron teeth that are integral to the gear itself. Those teeth can get to look like the starter ring gear teeth on a flywheel when they develope a burr on the edge of the teeth when the drive fails to engage properly, making a grinding sound.When that happens the starter drive gear can't pull out of the flywheel when the engine cranks or starts to run.This can happen to transmission gears in rare occassions. I would take a second look at ALL the syncro components in the 1-2 synchro assembly. I think you could get a good look just by pulling the side cover,but you may not be able to make a good check to see the actual engagement fit of the brass ring to the cone of the actual gear. The brass synchro ring should not totally butt up to the teeth on the actual gear. If that happens the ring has no ability to stop the gear effectively,especially at speed.This is why Mopar 4spds were slick shifted for drag racing, eliminating the synchronizing effect,but not at all street worthy, just power shifting. Bill