'70 Dart 392 Project; what direction?

3/16" dia steel lines.

My 68 has 73-75 a-body discs on front, 10x2-1/2 tear drums. I've tried 15/16, 1-1/32 and 1-1/8 MC's. The smaller one had the best overall feel. I switched to an aluminum 1-1/8 and hated the very hard pedal. I would sometimes use both feet on the wide brake pedal. Now I have an aluminum 1-1/32 and it's a good compromise.

In your 4 wheel disc setup, I think I'd try the 15/16 first. Discs are not self-energizing and you need the advantage of a smaller MC.
73-75 A-body power brake MC run manually.

Good time to install an adjustable proportioning valve in the rear line.