I have done it two different ways, I found some D18 shifters. In the picture above I removed the shift rails and welded the slot closed so the were solid at the end. then I used the grinder to cut a slot into the inside of each rail. Then the shifters just bolt on. Make sure to remove the interlocks so that you have front dig.
Here is a picture where you can kind of tell what I did,
I also used the same D18 shifter to use with the doubler set up. It let me relocate the shifters to where I wanted them.
My orginal plan was to fab new shift rails out of .500 round bar, but instead I cut one end of the round bar to fit inside the stock D300 shift rail slot. I cold then bolt on this "extension" to the shift rail. On the other end I welded a pivot pin. This pin goes through the bottom of the D18 shifter and is held in place with a c-clip. I fabbed a bracket to mount the shifters in my chosen location (off the tranny). and I was done.
The $129 twin stick set up out there is way overpriced
source a d18 shifter and you are a couple hours into it.HTH
