1971 360 no dipstick

I've never heard of that either, but if the casting boss is there, I would just drill an appropriate sized hole there and install the tube.
But if the boss is not there either,then IDK.
I have seen the dipstick hole filled, and painted over.

Since the pan has to come off anyway to remove the drill shavings, maybe you will see the evidence of the hole from the bottom.