Young kids in classic cars...what do you do?

I want to take my 7-year old for a ride in the newish '70 Swinger but I want to do it as safely as possible. She uses an "under the butt" booster seat in our other cars. Since the booster is designed (I think) to raise the kids up to the proper height for a shoulder/lap belt, is it better to not use a booster if only lap belts are involved? It seems to me that the lower the kid is positioned strapped to a lap-only belt, the safer they are. Also I figure for a single kid, the center rear seat with lap belt is the safest, away from metal doorskins and glass.

This all just feels so counter-intuitive to me after years and years of car seats/ carriers/ boosters that I wanted to reach out and see what others' opinions are. I want the kids to have fun but also want to be as safe as we reasonably can.

(And for those wondering, I am old enough to remember going round town in NYC traffic in my Dad's Duster- in the front seat with absolutely no restraints involved! But times have definitely changed since the early '80s. :) )