Problem with disc brakes after swap

You need a smaller bore MC. I also have 73-75 discs on my 68. I have tried MC's with the following bores, 1-1/8", 1-1/32 and 7/8" with no other changes besides the MC.
The 1-1/8" MC was just too hard. The car would want to creep at a light unless I used both feet on the pedal. I currently have an aluminum 1-1/32" bore on it now (vintage Direct Connection) and it is extra firm but manageable. IMO, the perfect MC for you is 7/8". It gives a nice linear pedal without a lot of effort and was my favorite but I try to save weight whenever I can, hence the aluminum on there now.

I intentionally left off the factory proportioning valve and safety switch and just plumbed in an adjustable prop valve in the rear line. (Summit but looks just like the Wilwood) The safety switch just turns on the red dash light when there is a leak in one of the two circuits. Believe me, when that happens you don't need a light to tell you something is wrong. My Neon did it just last week. (rear wheel cylinder went bad)

Order a MC from a 73-75 A-body. Not a manual disc car but a power brake car. This will get you the 7/8" bore and you will run it manually using a manual brake pushrod. Try it, I guarantee you'll like it!

FYI, you can measure the MC bore without disassembly because you can see the bore on the back side where the piston goes in and it's easy to measure.