I think the 3/4" rear bar would compliment what he has now. Those cars really needed a rear bar for balance.
I had the 69 RR large factory bar on the front of my hot rod. It didn't like turning with enthusiasm. Adding the rear bar made a big difference. I like it much better now.

I would add the Firm Feel bar to what he has. The biggest thing he could do to increase the stability of the car would be adding subframe connectors. That chassis is floppy! Tightening the chassis would make a night and day difference.