Well, as most everyone here knows already, I'm don't care much for engine swaps in Opels, BUUUUT (I digress!), if you're gonna swap in an engine that's making under 300 hp, the 9" Ford is pretty much overkill. It's also physically very large, and would either require cutting the floorpan up substantially to allow for suspension travel or jacking the rear of the car up a LOT. I think an 8" Ford or Toyota rear axle might be a better choice, lots of options out there for gearing and limited slips.
Watch out for that 6-speed tranny swap too, those suckers are physically very large, and would probably not fit in a GT tranny tunnel (I assume you mean either a T56 or ZF 6-speed?) So at this point, we're talking semi-tube frame and custom sheetmetal for the floorpan! You're possibly better off buying a kit-car chassis with a stronger suspension, bigger rear axle, and bigger brakes, then installing your driveline into it, and just dropping the remains of the GT body over the chassis. Otherwise you're going to have to reverse-engineer every part of the GT to fit the bigger rear end, bigger engine, bigger brakes, stronger front suspension, 6-speed tranny, rollcage (it will probably need it), improved cooling system, etc.
Just some food for thought, in my 11 years of running an Opel shop, I only saw 6 complete GT's with nicely done driveline swaps, and probably saw about 100 or more go to the crusher half-finished....
Bob