The new bracket rough measurements is about 16 inches across the the front and rear of the seat. The FJ60 has about 15.25 inch center to center for the bolt holes in the seat bottoms. The new seats have a closer to 15.75inch measurement from center to center bolt holes in the seat bottoms.
The metal brackets that come on the new seat are only 1/16" thick. These look like the typical slider brackets that are a C/W channel type rail system. (if that makes sense).
I went to my steel supplier to get the seat brackets made for my 60, and they only charged me for the labor of "fabbing" them up. I didn't pay for the materials. Here are a couple of pics. I made mine 2" tall so I'd have a more comfortable seating position with my long legs, but I'm sure you could have them bent shorter as long as you have clearance for your fasteners. As far as metal thickness, I think they used 1/8" mild steel, and it holds my 215# fat a$$ just fine.
Thanks for updates. I picked up some 1/8 steel at home depot. The summit seats I got are taller so I need to go a different route so I don't raise it with new the fab bracket. I'll post pix or start a new thread.