Go to the 'Forums' section, and then go to 'Performance', and then to 'Engine Swap Forum', and read the threads on swapping an engine into a GT. You can read on all the plusses and minuses of swapping a V8 into a GT. Yes, it's been done in the past, but the success rate of most people who set out to put a small block Chevy in is around 5-10%.
It's a lot of work, and will require another tranny, another driveshaft, another rear axle, bigger brakes, a much bigger radiator, a different front spring, cutting the firewall, custom engine mounts, and installing a rollcage. Why a rollcage? Every V8 swap I've seen that did NOT have a cage broke windshields every time they stomped on the throttle, or popped rear windows out of the body...the chassis will flex....a lot.
But if you are inquiring as to whether there's a kit or instructions on how to swap a Chevy V8 into a GT, there's not. The most common swaps seem to be the GM 60 degree V6, or Mazda rotary engine. There IS a substantial amount of information out there on either of these swaps. I suggest you do your homework and read up on them all you can, because it will prepare you for the kind of work you may have ahead of you.
While I don't necessarily care for engine swaps in Opels (hey, it's a free country!), I can appreciate a well-done swap. A good swap will require more work in planning before the swap starts than the swap itself takes to do. If you take the time to do it right, then it'll be worth it, otherwise you'll probably lose interest and the car will go to the crusher one day half finished!
Uh, yeah, to both the above posts. I was told recently I've been doing this V-6/T-5 mod for about a year now. Come to think about it, it's been longer than that, and I just got the engine in last weekend. I won't count 3-4 months on getting the rear-end, because I got 2 wheels polished and working on a third while it came from Germany and got built up locally. The planning, the checking out everything, the total exasperation when the engine would not run on the stand. The money spent for test equipment to find out why. The hours researching the net to find answers. But the sense of accomplishment when the engine finally lit off and ran. And when all the fabrication and cutting was close enough to put it all in the hole. GREAT. I won't take umbrage and Bob for not liking engine swaps too much, but, isn't a 2.4 a non original engine in a GT? Actually, my reason for the swap is not to go any faster than the car originally went, just get there quicker!!!
My 2-cents worth. Serious on a V8? Get a small car FRAME with the same width as the Opel (frame can be shortened). Strip the Opel's undercomponents, weld body pad mounts to it and start measureing. Would take some good thinking but you would have a serious strong frame. Personally, a V6 with extra axle weights is plenty of power in a light car for the street. If it's a V8 strip racer your after, a uni-body won't compare to a tube (round/sq.) frame.
There are very very very few cars that are narrow (GT narrow) and can handle the V8 torque.
A rolling tube chassis, with suspension, basic crate engine and drivetrain, tires and wheels, costs about $8000. That's nothing fancy. There are a lot of parts to buy and assemble. If you plan on using old/used parts, it's not worth the effort and won't save that much.
If it's really "your dream" and something you have the money, time and skills to do, then go for it. If it's a wild hair on the spur of the moment, stop now. Using great parts the car's final value may be $10,000.