Can’t get 1968 BB to crank over

You either need a helper, or some wire, so someone can read the meter or watch the test light. "Rig" ANY wire you have including an extension cord so you have wire to reach from the rear battery to the front. Now you can test various places.

First thing I'd do is do that or rig another way to read the meter, and clip it across the battery. See if you have voltage at the rear when you jumper the solenoid. Don't rule ANYTHING out just because it looks clean or is new. As said, battery may be down

Oxide on battery posts or clamps.
Faulty connection(s) from clamps to cable.
Don't rule out a defective cable. Might have been stressed in manufacturing and internally broken.

Might be worth it to drum up a GOOD set of jumper cables, and jumper a known good battery direct to the block and to the starter stud if you can get there.