Rear coil spring help

I have searched and cannot find a comparison for reference. However, I have found some useless information. So, I'm chasing this rabbit down the hole and coming up with conflicting info.

I'm currently running stock (ZW7) springs w/ a 1" spacer, Bilstein 5100s all the way around w/ the front set all the way up (about 1.5" lift). The truck looks perfect unloaded with about 1/2" of rake (as measured at the fender wells). The problem I'm having is sagging when I'm trailering (SxS or bass boat). I originally wanted to add Air Lift bags, BUT I now think I want to run a heavier/taller spring and/or remove the spacer just to have less BS. I'm not concerned about ride quality (my other vehicle is a dually).

I looked on GM Parts Direct and found the 4 part numbers available for OEM springs for my year model.

23154237 is for the ZW7 w/o MRC (smooth ride package)
23154236 is for the Z71 w/o MRC (off road package)
23152359 is for the Z95 w/ MRC
23154240 is for the Z85 w/o MRC (increased capacity package)

Common sense would tell me that I should probably go with the Z85 spring, but I found a thread on here stating basically that the Z85 springs suck and to go with the Z71 springs.

In looking up the specs on all four springs, I've found more conflicting info:

Free height Installed height spring rate wire diameter # of coils
ZW7 16.18 12.08 251.25 .54 9
Z71 16.47 12.079 251.25 .551 9
Z95 15.59 12.08 243 .64 8
Z85 15.12 12.08 256.95 .63 7

As you can see, the Z71 has the tallest free height, but the same installed height and spring rate as the stock springs.
The Z85 springs have the shortest free height, but the highest spring rate.
The Z95 springs have the largest wire diameter, but the lowest spring rate (probably because the MRC shocks take up the slack).

Which springs are installed on the Max Towing Package?

Thanks in advance!

rustyj671 said:

Which springs are installed on the Max Towing Package?

Click to expand...

Can't answer this directly, but, if you collect several VINs of the vehicles you're curious about,
and plug them into GMPartsGiant.com's VIN decoder
(guess where the VIN goes)
you'll eventually discern what package or packages tend to go with what springs,
among many other coincidences.

I found where the Z85 springs are associated with the NHT code. I'm thinking these will accomplish what I'm looking for except losing the spacer. As long as this damn thing doesn't look squatted when I'm towing, I'm happy.