For seat mounting, the absolute simplest thing to do--and waht would be a good interim solution until you decided to get more fancy--would be to take a piece of 1/2" thick MDF (say 20" x 22") and bolt it directly to the seat rails. Then take the stock momo sliders and mount them to that.
I know you need 5" under the seat so that might not be the best ultimate solution. So, once you figure out the width of the mounting holes of the sliders, you could then adjust the length of the cockpit cross members to make the pedal rails the right width across. If the seat sliders have bracket/feet that are 3.5" tall, then the rails woul make the seat bottom at least 5", not including the side mounts.
I know you need 5" under the seat so that might not be the best ultimate solution. So, once you figure out the width of the mounting holes of the sliders, you could then adjust the length of the cockpit cross members to make the pedal rails the right width across. If the seat sliders have bracket/feet that are 3.5" tall, then the rails woul make the seat bottom at least 5", not including the side mounts.