Something you may consider is utilizing the HDK (100%) bolt on 64-66 A body upper shock mount / chromoly support hoop (@$349 plus shipping) that will allow for a longer travel / longer spring. Although sellers of the short shock packages tend to disagree, IMO the longer the travel, the longer the spring.....the nicer (less harsh) the ride. The HDK upper shock mount / chromoly support hoop modification will also allow you to move your hubs inward (easily 3/4") IF you are willing to shorten your upper and lower control arms. I have several builders that have done this mod with very positive results.
I'm curious to much are your springs collapsing at installed ride height?
Feel free to call to discuss,
Denny
HDK
304-939-0097