You don't need to do that
I just fitted my MK3 seats a couple of weeks back to my 92 WT Hardtop and didn't require a bracket/flatbar for either the drivers or passangers.
On the passanger side just use the original sierra rails.
Use the first bolt (closest to glove box) to "lineup" the seat to the rail. You will need to use the GTi bolts as the sierra bolts are too small so therefore you will need to 'elongate' and make the hole bigger (you will need some washers incase you make the hole too big)
I then lined up the rear hole of the slider onto the seat and roughly drew where the hole will need to go. The GTi seat has fabric on there so just cut that away nicelly and you will find a hole which is 1-2mm off where you require it to be. Elongate and make that hole bigger...a washer and the screws from the GTi seats and you're set
Don't make too big of a hole or have a huge head for the nut as you won't be able to slide your seats. The one that bolts to the floor will hit the one that screws into the seat so the seat won't go all the way back. (might not apply if you use the GTi original screws but i found that to be the problem when i fitted the autotechnica seats so thought id just point it out)
If that doesn't make sense and you need to have a look just give me a buzz to organise a time