Ford E-350 and 450 seats are composed of two parts, one is the box frame that is anchored to the floor and the other is the unit were the sliding rails are attached to the box, get some steel flat bar 3" wide by the amount needed for the two sides and a minimum of 1/4" thick- 3/8" will be better, measure where to attach the bar to the floor box and then calculate how far back you want the seat at the back end of the slide to be and mark the hole centers, you might have to get a machine shop to drill the holes and sand the saw cuts for you, then attack the bars to the seat and to the floor box.
Make sure that you have the correct grade bolts, washers and nuts, the bolts should have 5 small raised lines from the center to the perimiter, do not use any Chinese junk!
I have the E-350 and raised the seats 3" up and 4" back using square 1/4" tube, it can be done.
navegator