I am in the process of converting all engines to the fixed pilot, full length rails versions for 3RS... it is worth it. I've been able to make a pilot filler, make new handrails, and a pattern I use for drilling holes in the pilot to line up with the holes in the body to secure the pilot. There are small holes for the kadee couplers you can also use with a #2 sheet metal screw.
take a look. I also made a cover plate that goes around the draft box to cover any unwanted "holes" where the 3-rail coupler went. The handrails are .025 brass wire from Tichy Train Group that I formed using another 2-rail loco I own as a guide. These look great compared to the 3-rail versions....
The gap is 4mm between the bottom of the frame to the top of the pilot. (distance between each step on pilot)
It does take time to do, but worth it. I've done 8 so far... on this particular unit I also added cab window shades that I made and filler between the top of the truck to the frame.