I don't have an answer as to why they decided to use a ground controlling regulator in '70.
You certainly can use the feed controlling regulator as you plan.
Just be sure to charge the car battery on a charger. That will reduce the electric load on the new alternator, the old RV, and be better for everything.
Especially because many rebuilt/repaired alternators, especially squarebacks, have a higher 'rating', and may draw more field current under high demand.
A discharged batttery is one of the highest electric demands placed on stock system. Charging Battery with Alternator - Warning
