86 SVO Rear/A-Arms on a 91 GT?

It will bolt right up. I know someone with a GT that had the complete SVO conversion. The only problem is if he kept the long axles, (1.25" longer than stock) then the wheels are a bit far out in the back. You will need the SVO MC and PV to have it balanced right. The A arms should be fine also, just need to know if they are set up for stock type springs or coil overs. The brakes may need an adapter for the rear line and you will need a conversion for the fronts to the MC, along with SVO or Ford Cobra kit ebrake cables. I have the same setup in the rear of mine but with modified brackets to use the stock width narrower axles.

See http://svo73mm.cjb.net for more brake upgrade info.

Good luck!