Hey,
You are currently using longer shackles and air shocks , is that avoid contact between your wide tires and the wheel houses?
I've never been a fan of the use of air shocks or longer than stock shackles to correct ride height ,wheel hop or handling issues. What I do for myself is start with known good springs(primarily the main leaf) I take apart the actual leafs, I then swap in different leafs to get the desired ride height,handling, wheel hop correction,etc. It's a bit labor intensive and you need donor leaves. I have on a few occaisions gotten lucky and bought a car with good rear leaves or I bought springs that were perfect right off the bat.
My thought is that the fifth leaf should not hurt,but you may be better off with a higher quality 4 leaf setup.My main recommendation to you would be to talk to the company that you are thinking of buying the springs from and find out what they suggest. I've seen where guys just pick up a catalog and order what they see,they then install them and end up having to take them apart or pay a suspension shop to fix the issues they may have as far as ride height and such.
I recently did some work on a 70 that had cracks in the rear upper shock mounts. Most likely as a result of air shocks, air shocks have a tendenacy to place too much weight on areas not designed to handle it.
If your main leaf looks like a handle bar mustache with the air shocks deflated you should swap them out.
I recomend this site for tech and possibly purchase. These guys designed and built the springs for just about every american passenger,sports and muscle car ever made.
Eaton Detroit Spring Home