My plans for my 69 Dart Swinger 340

Why disc brakes? I have used 10" drums and manual steering for years, no issues with either. Only discs you can use are 67-72 K-H 4-piston if you want to keep your small bolt pattern...
I believe (and I bet that most on the forum will agree) that disc brakes are a big improvement over drums. They will stop a car in a shorter distance. I added disc brakes to my 69 340 4 speed Barracuda, and I think it is the best upgrade I have made to it. Now I will agree that drum brakes do work fine, but disc brakes are more efficient, they stop better and they don't fade with heat as badly as drums. And if the Op does not mind using aftermarket disc brake upgrade kits, I used a Stainless Steel Brake Corp Disc brake conversion that allowed be to keep my SBP wheels. There may be other kits available.

That being said, I agree with you 100% about keeping the Manual Steering. I have PS in my 69 Barracuda, and it is a bit sloppy. I have replaced everything, and it is still a bit sloppy. I have seriously thought about going to manual steering a lot! Manual steering in an A Body is not a problem.