All my comments here are directed at improving handling. Personally I would not bother with the larger rear end until or if you ever go to a V8; the smaller rear should be quite adequate with the mild and wilder /6 engines you list, and will be lighter, which means it will handle better over rough surfaces, due to less weight for the shocks to control. The posi wil lbe a big improvmewnt so go for that. Use high quality gas shocks like Bilsteins. Similarly, I would use AL wheels, not steel; the weight difference is huge for rough surface handling. Mono-leaf rear springs would be a good improvment but that IMO should come last unless they are cheap. The front torsion bar spring system is pretty good so I would not touch that.
The use of just a heavier sway bar is typically not optimum for handling; production car designs put the front sway bar in first and heavier, so that the front end will push out in hard cornering before the rear end gets loose That is best of the average driver who can't anticipate and handle a loose rear car, but is not optimum for handling. So a rear anti-sway bar is in order for best handling. (As a later item.)
Brake combo of disc front/drum rear will be adequate for all but the most demanding paved surface driving. The largest drum brakes are best, simply for heat dissipation, and using a good semi-metallic compound up front and a heavy police interceptor level of rear shoe compound will be good. Brake bias can be adjusted with compound changes if you ever get to that, but most people don't go there for street driving, unless they have a lot of rear lock up in wet conditions. So that's ususally not too important.
The one drivetrain comment I will make is on the trannie; if you ever could get the 4 speed OD trannie that came in the Dart Lights and Plymouth Feather Duster, you would be really happy with the gas mileage increase with the 0.83 ratio OD 4th gear. There may be other good trannie conversions out there with an OD; I am not up to speed on those. If you are out for spirited street driving, a manual will add a whole new dimension to that.
Good luck! Sounds like fun!
(BTW, my race backround it rally, and we put in all sorts of effort for rough handling improvements; those same things make a car very predicable and allows much more aggressive handling for street use.)