Well, you can make the car handle better and drive like a newer car would without spending the money on a coilover conversion. There's nothing magic about coilovers, they're just a spring and a shock. There's no dramatic improvement in geometry or handling just by going to coilovers. You can add bigger brakes with stock spindles, there are quite a few options actually. Now, the rack and pinion is something you're not going to get without the coilover conversion, at least if you want the geometry to work. And of course headers are a tight fit for torsion bar cars, those are legitimate reasons that you can't really address if you stay torsion bar.
As far as the weight savings, it really depends. If you compare like-to-like, ie, manual steering to a manual rack, the weight savings is not extreme. Comparing a stock manual steering set up to an HDK coilover conversion with a manual rack, the weight savings is only about 31 lbs. Now, if you compare an OE power steering set up to a manual rack HDK, that's like 72 lbs. That's using all stock stuff too, you can knock some weight of the torsion bar set up by using tubular UCA's, adjustable strut rods, etc. You can see a weight comparison I did using all the HDK numbers here
Anybody running the RMS AlterKation? Was it really worth the cost?. It covers everything including the difference between a power HDK and a Borgeson power set up for the stock K, that just falls in between though.
Now, the beginning of that thread has a whole lot of mess going back and forth between what set up is better, coilover or torsion bar. There's good info in there, and a lot of opinions. And that's pretty much how it always goes. The fastest Mopars on the AutoX and road courses are
still torsion bar cars. And you can significantly improve the handling of the torsion bar suspension for about half of what it costs to do a conversion. No, you still don't get the same feel as a rack and pinion, and you still have to snake the headers through. For me, that's something I'm willing to do for a few grand in savings.
As for the coilover conversion, I would say call HemiDenny. I think his stuff is the best bang for the buck, and he's super knowledgeable about suspension design and his product. The RMS is another proven system, but it's pricey. There's a few folks that really like the Gerst stuff but I'm still not completely sold, at least not compared to the HDK. I would
not run the Magnumforce. And of course, full disclosure, I don't run
any of them. I run a torsion bar set up, although it's about as close to 100% modified as it gets. And I will totally admit being biased toward the torsion bar suspension, that's my opinion.