I just went from 4 leaf standard eye Grab-A-Trak (with KYBs) to 5 leaf reverse eye TCP (with Bilsteins). Other than lowering the rear a good 2", the ride is only a bit stiffer...but I need the control for the track. My car also has a Watts link that controls the rear end to the 9s...so you need to add that to the equation.
I'm of the belief that each vintage car reacts differently to identical setups, but that's just me (and my car is possessed, so take that with a grain of salt lol). You may just have to bite the bullet and buy a new set of 4 leafs and see if you like the difference.
1966 Restomod/track car, 1973 Q-code Mach 1, 1997 F250 diesel, 2010 Crown Vic (ex-City of Lakewood PD), 2015 Focus ST-2