gene has a good idea that is super simple to fab if you lack many tools. another option would be to make a riser from 12g. strip. start by detirmining how high the riser needs to be. include in this measurement an additional 1 1/2-2" for the bottom flange and 1" for the top flange. form the riser to fit the floor either as a single [both seats will attach to the top, and the riser will be clearanced to fit the trans/driveshaft hump] or as individuals [each seat will be attached to it's respective riser, possibly saving fab time for the hump]. bend the top and bottom flanges 90*. for the single riser. you will then have to make a top that will support both seats. start by cutting 2 pices of 1x1x 1/8 wall square tube that will attach to the front and rear walls of the risers. these will be positioned where the inboard mounting legs of your chosen seats will attach. be sure to center these pcs. on the mounting bolts. the outer legs will mount to the upper flange. drill mounting holes as necessary, but be sure to use spacers in the inner tube suports to avoid crushing the tubes when tightening. an easy way to fab the tubes, is to use common black pipe.[3/8 or 1/4", depending on your bolt size. first drill the proper size holes in the appropriate places. next, measure the od. of the chosen pipe and drill the TOP holes only in this size. cut the pipe into 7/8" long pcs. this will insure the spacers will be flush with the top of the square tubes. insert and tack to the tubes top surface. as for the attaching nuts, you have 2 choices. first, you can just use the appropriate nuts-n-bolts and bolt up as usual, or you can weld the nuts to the bottom of the tubes [as i would do], allowing you to only have to use 1 wrench to tighten.
next, measure and cut your 1x1 tube to fit side-to-side between the front-to-rear tubes. this will usually consist of 2 long and 1 short pc. remember to make sure these tubes are at the same surface height as the front flanges. weld as necessary.
next is the top surface. since there will be less structural strength needed on the top [due to using the 1x1 tube as central mounting points] you can form the top from 16-14g. flat sheet. if you feel uncomfortable whith this choice of material, use 12g. or 1/8". this will only add unnecessary weight[in my opinion.] i like to put 5/16" holes on 1 1/2" centers above the 1x1 tube and plug weld to the tube in addition to welding the outer perimiter of the top plate surface. you are now done !
as for the dual risers, you can form to fit the seat mounting bases and use the upper flanges as mounting points for the seat bolts, therby negating the need for the 1x1 tube.
perhaps one of the guys can sketch out what i have just described ? i would if i knew how.
i think the above words are pretty self explanitory if you think about it a little bit. hope this helps out a little.
