Upgrading a Fairly Stock Suspension

So after school's over and winter break starts my friend and I should be able to fix all the safety issues on my 1968 Dodge Dart GT with regards to the steering coupler and brakes. I was thinking my next step on the Dart would be beefing up the suspension a little because it's a bit warn out.

My friend has a pressure washer so the first things first is we're going to clean up the underside of the car of all the grease. A couple of bushings may need attention, (that my friend may already have) so we plan on possibly replacing those as well. My friend and I also plan on fabbing up some sub frame connectors for the car (don't know when or how soon, but he's already done this on his Demon), instead of buying a set.

As far as I can tell, the entire suspension system is stock (no sway bars) except for the air shocks in the back (that don't hold air). The rear leafs are a little saggy (and just look puny). I'm currently running on 14" wheels with 205/75/14 tires; however, I don't want to change those out till I can afford an entire bbp swap. But when I do, I would like 17-18" wheels with 275's all the way around. And lastly, I'd like to stick with a K-frame and leaf springs over the alter-K systems and 4-links (they just cost too much).

I want a car that feels nice and firm around the turns (a corner carver I can take to through the mountains or on a track/autocross once in a great while) but still nice enough to comfortably cruise and not feel every bump in the road. Kind of like the Green Brick and Tim Werner's Valiant.

I know Hotchkis, Hellwig, and Firm Feel are all great suspension companies. Hotchkis is very local to me (and currently has a 20% promo code) and Firm Feel has pretty good prices as well. I obviously can't afford to replace everything at once, so this will be a piece by piece build up.

So where should I start? I was planning on getting some good shocks and probably a tubular sway bar first. For shocks I was thinking either Hotchkis's new Fox shocks or Bilsteins from Firm Feel. And for the sway bar I was leaning more towards Hotchkis or Hellwig (need more info on the tubular Hellwigs though). Would it be ok if I mix and match suspension components from both (or more) companies? All input, formulas, and information is greatly appreciated as well as any articles you can find on the web. Thank you!