An 1800E or ES is the best source of dual-line front calipers. The 1800 front suspension is identical to the 122, so the calipers and disks would fit your front-end.

You still have 2 problems, however. To have a true split system, you would need to have brake boost on each of the circuits, since each half of each front caliper is separate. The "simplest" way to do this is to adapt an 1800E or 140 brake booster/master cylinder assembly so that the brake booster operates on the master cylinder, instead of being plumbed in after the MC.

The other problem is that you will have to divide the rear brakes so that each rear wheel is paired with one of the front circuits. Otherwise, if you loose integrity in the circuit the rear wheels are paired with, you'll only have brakes on the front wheels, which could be dangerous. (Admittedly, this is no worse than you have now with the '68 split circuit system).

I don't see any problem using rear drums with the 1800 front calipers. You just want to use some kind of proportioning valve like the 122 already has.