My 70 Challenger 340/4 speed is very hard to get into reverse, all other shifting is great. Any advice on where to start on repairing this problem is appreciated

This one is easy, the side plate on the tranny needs to be loosened up, then you tap the rear/bottom portion of the plate UP--it will move a tiny bit, less than a 1/16". Then tighten the plate and try it.

The clearance, internal, for the reverse arm is very small, and is known to bind against the case. Very common

If you have not done it in a while you can adjust the rods, per the FSM.

Start with the basics, air gap on the clutch disc

Is reverse hard to find with the engine off too? Or are we grinding while trying to engage reverse?

Are you running the bell housing that it left the factory with? If not is it a replacement or aftermarket?

Are you going from N to reverse,if so try going into a synchronized gear then right into reverse. sometimes you need to stop the input shaft esp if the clutch has a little drag going on.