1970 Dart starting issues- key will not crank now

Bad grounds will plague you to the end of time. Sounds like the steering column isn't grounded. There is a little wire with 2 star eyelets everyone let's fall off when they remove the colums. So check that.
Also a ground up on the radiator support by the horn. Check that. The terminal that is a 45 degree angle on the starter relay is the NSS terminal. Check that.
And bulkhead are Terrible on these cars. Yes you can inspect those too.
Good things to check. Even seen battery terminals loose over the years