rebuild and box the lowers ya have already. being a 74 i'm assuming it has disc brakes already? if so then you already have the large ball joint upper arms so just replace the bushings with moog offsets and replace the ball joint and be done..:)
This. Unless you're going to run REALLY low with the torsion bar suspension, the tubular lowers aren't really necessary (they do add some additional suspension travel for lowered cars). Boxing the stock LCA's will be fine, and will allow you to address any play they might have developed. Jim Lusk has a great video on YouTube on how to do this.
For the UCA's, rebuilding with a set of Moog offset bushings will be enough to modernize your alignment specs, and will get you enough caster for a street car. If you're planning more of a corner carver, the tubular UCA's can get you more positive caster than the offset bushings will give you with the stock UCA's. But, really unless you're running 17" or larger rims with more modern tire compounds you won't need that much extra caster. If you are planning on 18" rims, the tubular arms do allow more rim to UCA clearance, which can come in handy for running really wide rims up front (18x9 or larger), but anything less than that doesn't really need that either.
So my vote is, as usual, depends on the application. For a basic street car with 15" rims and BFG T/A's you can keep the stock stuff and just reinforce the LCA's and run offset bushings in the stock UCA's.
