Aftermarket brakes for a 70 Dart?

there's a lot of considerations here. expense, PITA factor, and availability among them.

how much do you want to spend? do you care if your rotors and calipers are all flashy because they can be seen thru the wheels? do you drive enough to possibly need consumables on a yearly basis or maybe on road trips? do you care about availability 2, 5, 15 years from now? is the cost of those parts a concern to your budget? do you have infinite free time to hunt down all the parts and bits and bobs and whatzits when cobbling together a system of hodge podge parts? are you okay with your car being down or waiting weeks for something that's not in stock or currently unavailable?

for all intents and purposes, a stock KH or 73~76 single pot is going to be more than adequate for your intended application. and going KH BBP on the front is probably the "easiest" way forward.

personally i'd swap it out to 73~76 single pots. the "small" slotted/drilled rotors with aggressive pads will stand up to occasional track use and be great on the street. yes, the price on the front end is higher; and yes, that is more work involved. but, the parts availability and pricing is hard to beat-- and the PITA factor is basically nonexistent.

best of all, you know that they'll fit the centerlines you plan to run.