Dakota -- Whatever you end up choosing, you'll notice a major difference... stiffer torsion bars especially (don't be afraid of larger TBs... 1.10-1.12--1.18 good for street). Since you have a BB engine, you want to reduce the lift and dive of that extra weight.. consider ~1.18 (from Firm Feel) and be done with it. BUT, mark my words, you will soon notice that the larger TBs aren't bad at all... and that you could've gone larger (I run 1.24" diameter... largest that will fit inside the factory hex opening... and I use my car on the street/hwy.. and HPDE/HSAX/pylon AX).
Hotchkis has a proven package, although their TBs are, in my opinion (and other's that I've spoken to), too soft...should be at least 1.18 or 1.20. Whatever TB you install, you'll soon realize that its not too harsh and you could've easily gone larger/stiffer.
Leaf springs -- step up the stiffness.. and keep them near flat when loaded for your cruising.. maybe just a slight arch. Firm Feel, Hotchkis.. both are good choices. You'll end up with a slightly lowered rear, but that's OK... just lower the front slightly with the TBs. (Note: check the driveshaft angle... shims for angular correction may likely be needed). You can also adjust the rear ride height with different front hangers... see "AR Engineering" selection (he offers them through different vendors, Mancini, etc... and direct from him).
Upper Control arms -- Hotchkis has their unique design... very good.. and adjustable for camber/caster. SPC brand/design is also good... fully adjustable. QA1.. depends which design you get. Firm Feel... non-adjustable but designed for much more increased caster and some negative camber. Regardless what you get, adjustability is desired so to get much more positive caster as possible... with a little negative camber.
Lower control arms -- yes, you can have them "box welded/stiffened"... your choice to do this.
K-frame welding -- yes... Firm Feel offers a kit and service ... your choice to do or not.
Sub-frame connectors, etc -- yes... much improvement will be noticed... plan to get it done... multiple choices out their... ground clearance can be an issue pending design. I suggest looking at other e-bodies for their setup of SFCs... and then decide... $$$ is factor as well as installation skill.
Heim-joint setups for struts and tie-rods (Hotchkis, QA1, PST, others) tend to have a shorter life-span vs traditional.... when driving on the street...so.. avoid rough bouncy roads, pot-holes, etc. Firm Feel has good choices for these (adjustable, heavy-duty and polyurethane... that's what I use).
Sway bars... get the now common 1.25" hollow front; any rear will be good, but look for at least 7/8-13/16" diameter. IF you run a rear disc setup, then you may have to avoid the factory above-axle (frame-mount) rear sway bar as it could interfere with the discs (the Dr Diff 10.7" discs hit the factory brackets at the leafs... as I just discovered... so then an under-axle rear bar will be needed (I went with the Hellwig bar.. 7/8" diameter and 3-way adjustable.. and less costly than the Hotchkis rear bar).
Shocks -- you get what you pay for. Today, there are good choices of double adjustables.. Viking, QA1... both good... same technology. I would only suggest (2x adjustables)... so you can truly fine-tune the ride and very easy to adjust! I've recently switched from red Koni D... to QA1 double adjustables... a noticeable change and worth the expense.
Rims/tires -- Be aware that as your sidewall becomes less (50-45-40-35-30 profile), the ride quality will become harsher.. and also more likely to damage the rims if/WHEN you hit potholes in the road. You'll learn to adjust you tire's PSI for ride quality... just don't go too low (not less than ~28 psi) or you're asking for possible trouble with the potholes and rims... Newer cars (late model Challengers, Mustangs, etc) have lots of damaged rims! For your described driving, you won't go wrong with the YearOne 17x9 Ralley or Road rims.. they have the correct offset (zero) and backspacing to properly fit inside our E-body wheelwells without any mods (maybe just tucking in the lips if you want extra insurance from the tire's rubbing.. pending how soft your suspension is. Cudas tend to be a little tighter vs Challengers at the wheelwell/fender lips)... you could go 8" and be pretty much A-OK.... as long as the offset is zero (or no greater than positive 1/4"-1/2 at most). Tires.. typical 275-40-17 will fit with the YearOne rims. IF you go 18" diameter, then you have to be more cautious with rim selection.. and tires sizes... shorter/smaller sidewalls a critical issue. Another choice would be 16x8 rims (various choices) and use 255-50-16 tires (BFG G-Force Comp-2, decent sidewall height for the street/hwy)... mini-lites are nice!... this combo should easily work for you.. I've done it and like it very much for street/hwy.. and occasional road course lapping events... good ride quality!!!
Eventually, get some better supportive seating and a smaller/fatter steering wheel.
Enjoy the ride! (Where are you located???)