The springs on your '91 are longer than those on his 4Runner. FWIW, the factory springs from a 89-95 XtraCab work on some Tacomas (I think 96-97) and gives a little bit of lift and less sag.
OME suspension is nice, this is what I run, too. I started with mediums and added a leaf, but took it out. So I'm just running 6 leaf packs with a WilderNest camper. The reason you'd need the extra leaf would be if you carry a lot of weight and want to retain the full lift. That was why I had the 7th leaf, to be sure to clear 33" tires.
Now that I'm running 30" tires, I've never had an issue with clearance with regular medium packs at full payload (camper plus gear & passengers is 1,150 lbs). The OME packs have 3 main leaves and 3 progressive overloads, which is different than Toyota. They have 3 main leafs and one huge diving board overload. When you hit the factory spring limit you know it, the springs just about stop traveling unless you have a lot of weight in the bed.
Having that extra leaf in the OME pack made it ride pretty bad empty, which is mainly why I wanted to take it out.
'91 Pickup - Imelda
'08 Tacoma TRD
'09 Kawasaki KLR650
'12 Gunnar Rockhound 29
"To live is to suffer, to survive is to find some meaning in the suffering." -- Friedrich Nietzsche