The idea of being able to use the 67 control arm has merit from a parts installation perspective (disregarding suspension actuation, functional design, and theory which I will leave to the experts.
) The 65/66 UCA are the same length as the 67/68 stuff. The issue is that the 65/66 towers are 3-4" (or so) closer together than the 67/68. The swapped in longer 67/68 LCA would need to accounted for by mounting it even more inboard than what the factory 67/68 LCA frame mounts were designed for.
As far as steering parts, the 67/68 stuff would need to be swapped in to see the added benefits. No big deal as its a bolt in if you use the correct 67' pitman arm for the 1" shaft. Might need to experiment with tie rod ends though to get the correct lengths. I would also want to have adjustable strut rods to correct caster, but those are also available for a 65/66.
Not sure how the 65/66 spindles would come into play or if the swap to 67+ spindles would be a good or a bad thing in this odd ball combination (they have a relocated tie rod arm.)
It would only make sense if you went for the whole package: shock towers, control arms and steering linkage.
It would give a larger engine bay, better suspension geometry and a wider track. The better change in suspension geometry is probably more an optimization/refinement than a night and day difference and probably doesn't justify the cost. The wider track probably means that the wheels no longer fit, which would require wheels with more backspacing and consequently, you loose the benefit of the wider track (although the reduced scrub radius may be an advantage).
Your info is correct but I would like to add a little clarification. The major limitation of doing the whole package is that the 65/66 body width is narrower than the 67+. To make my "whole package" shock tower/suspension swap work I used the early narrow 65' style steel wheels (66/67 are "similar.") IIRC, they are 5" wide. The wider track is definitely still there but you can't install a wide tire due to to the wheel wells which have a smaller opening than the 67+. I use a 195R/75x14 front tire and the increased track width is still very evident when you look at my car. That tire works well for my drag car needs but wouldn't be worth a crap on a road coarse. Since my front fender wheel well lips are still intact, one might be able to use a slightly wider rim and a different aspect tire to get a little more rubber in there.
Wish I had a photo from the front, but all I have is a side view. I run 5* caster for drag usage so that explains why the wheel is pulled forward in the opening:
The front of the car also sits higher than what one would typically want. The new springs never did settle and that is being taken care of this winter.
PS, the signature photo is with the 65 suspension so its not a valid reference for the 67/68 conversion.