In reversing the 2 forward levers, do you mean by placing the 1-2 gear operating lever in the 3-4 levers place and vise-versa?
I made the linkage all from scratch. I got some stainless heim joints from Fastenal, some stainless tubing, and welded some SS nuts on the the ends of the tube where I used shoulder bolts that hooks it up to the operating levers. On the shifter side, I had my buddy machine 3 stainless steel fine thread bolts down to the inside diameter of the tubing, slid them in and welded them up...