Where did you get the 18-1/2# spring that you have now?
I am kind of learning these long slides as I go, I bought the long nose-6- spring plug got it to day and surprise!! the gun comes new with one in them. which means that springs for a 5 in barrel will work in them, that might save some one eles a bit of time and trouble. I love learning, and I love these long slides, thank you
Couple ways to deal with this.
One - You can get a custom recoil spring plug from EGW that allows springs for a 5" gun to be used in a long slide. Here's a couple links to those. Blue for standard guide rod -
http://www.egwguns.com/1911-parts/long-nose-6-spring-plug-blue/ And stainless for standard guide rod -
http://www.egwguns.com/1911-parts/long-nose-6-spring-plug-ss/
Blue for full length guide rod -
http://www.egwguns.com/1911-parts/long-nose-6-spring-plug-blue-with-.332-hole-for-6-guide-rods/
And stainless for full length guide rod -
http://www.egwguns.com/1911-parts/long-nose-6-spring-plug-ss-with-.332-hole-for-6-guide-rods/
There are many options in 5" recoil springs from 5# up through 24#.
Two - Wolff makes springs for long slide guns, but they do not list a weight, so you will have to experiment or find a way to measure the spring weight. You can do a search and find various home brew spring weight measuring devices that folks have made. The springs are listed part way down the page here -
https://www.gunsprings.com/COLT/1911 GOV'T PISTOL/cID1/mID1/dID1
I do not recommend using heavier than 20# spring in the gun as that beats up the slide stop pin and lower barrel lugs on slide closing. The recoil spring's primary purpose is NOT recoil mitigation, but rather feeding the next round out of the magazine into the chamber.
A far better way to approach this (and make an improvement in your gun at the same time) is to purchase a flat bottom firing pin stop from EGW and fit it to your gun. A little file work is all that is needed to get a snug fit. Kimber makes a good gun, but they cut some corners. The factory firing pin stops are incorrect as they are cut for the Series 80 Colt safety (Kimber's safety is a Swartz) and leaves a gap which can trap the tip of the safety plunger and shear it off during assembly/disassembly rendering the gun unshootable. Here's a link to the page -
http://www.egwguns.com/9mm/.38-super/.40-s-w/10mm-firing-pin-stops/ You will want the one marked Series 70. There are 4 listed - standard profile or with the top trimmed to clear low mount adjustable sights, and either type in blue or stainless. I recommend the blue as it seems to be more durable for firing pin stops. Fit that to your gun and you can use a lighter weight recoil spring (whether 5" with the custom plug or 6") than you can otherwise use. At the same time replace the mainspring with a heavier unit. Standard is 23#. Extra power goes from 25# - 34#. I would start with a 26# or 28#. No need to go higher.