Wilson Combat has a 4" recoil assembly for flat wire springs. Avail. at Wilson and Brownells Flat-Wire Recoil Spring Kit, 4" Compact / Professional, 9mm, Chrome Silicon, 13 Lb. Part # = 784Also for 38 super & 45 ACP kit and/or springs
Purchased the M&P 9L added Apex sear, striker block, Glock extra power trigger spring plus polishing.... 3 1/2 lb. smooth trigger. Added Warren FO Front and Tactical rear. All of my own choices and did not have to throw away perfectly good parts.