Cameron, Ron
You guys mistake my meaning. My car's brakes work fine, lock up, and stop the car within an acceptable safety margin. Actually the rears lockup a bit too much - making me think that the rear limiter needs to be replaced or rebuilt. I have a fine spare so no issues there.

Regardless that's not my problem with the original system.

By "Barely Adequate" I mean that a single-circuit externally boosted brake system is barely adequate in a modern world. There is almost no safety margin if there is a pinhole leak or a broken line. True that my 68's split-system is better than the old one-circuit, but it's still just one circuit in the front and one in the back. That's a marginal improvement at best, considering the bolt-on modern volvo improvement available in a 140 or later 1800 system.

My question is whether there is a physical compatibility issue between the dual-circuit 1800 calipers and brackets and and 120 series hubs and brackets.

Can I bolt on a set of dual-circuit 1800 calipers without changing my 120's wheel hubs and caliper brackets?