rear brake upgrade on 70 dart

Thanks a lot for the great info and advice!!! I am going to start with the rears. There is a leaky seal or w/c so i need to do the obvious first. I will see how it feels at that point and think about rebuilding the fronts as it has sat quite a bit.

More importantly right now, the thing isn't running right. It has had 100% tune up done including distributor, coil, wires, plugs, carb, valve adjustment, etc. The problem is a lot of little poppy backfires. When I drove it home it ran ok but I remember some of the little pops. Before I replaced the coil I moved the timing around because it was almost impossible to start. I finally realized the coil was roasting hot so i replaced it then it started right up. I had also in the process checked the compression and was good at 120-130. Anyway, when I went to drive it it after the coil it was popping a lot and couldn't get up. Under no load it would rev up and sound good and smooth it was just under load it had trouble. I don't have a timing light but I tried by ear in many different positions with similar results. My conclusion is that it probably needs a t-chain. My timing light is broken so I can't check for timing mark bounce. The only other thing I am not 100% on is the valve adjustment. I don't hear any clatter though so it is probably OK or at least not the cause of this problem. Any thoughts?

I also am trying to do some research for another car I want to build regarding rear ends. Probably a 63 valiant, I want a light, high-revving power plant in an extremely light car. I already have the motor I want in mind. My question is about the rear end. Because I am trying to keep the weight and rotating mass way down and for general simplicity I want to keep the 7.25 or something else that is as light. What I need to end up with is the 5x4.5 bolt pattern in a very light assembly that is maybe slightly wider than stock or stock width-- and the main thing is I want massive stopping power. I know about the front brakes from a 73 and newer . What about the back???? How can I get some massive brakes in a light but strong assembly. This is going to be about 300-350 HP but only about 180 torque. and 8000 red line. Oh, and it has to be disk because it has to look cool through the rims. Thanks!