You are correct with the need to have neg caster, between -2 and -4 will give nice steering feel.
The caster bar can pull this (basically you need to have the lower arm forward from the top arm), not sure how you checked it yourself, unelss you have a setup at home?
Best to get it to the guys that can do the adjustment, as it will effect everything and it's important to keep the camber correct also.
I was out of adjustment with my 67 (similar to the 70), so when I was replacing the top pivot hardware adjusted the top arm before bolting it to the body and got my caster back to -2.5 once I took it back to the alignment shop.
Bit more work but that's the fix if they can't get anymore caster out of it.