1) Remove the GT engine, tranny, and rear axle.
2) Hack up the firewall, make custom engine mounts.
3) Install 350 with appropriate tranny.
4) Find a radiator with double the GT's capacity that fits.
5) Find a front spring that will handle the 350's added weight.
6) Install much larger front brakes so the car will stop.
7) Install a narrowed rear axle from a Chevy or Ford...The stock GT axle will NOT last.
8) Make a custom driveshaft
9) Weld in a rollcage and make undercarriage frame connectors, otherwise the 350 will break windshields because of the torque.
10) Sell the car for $500 when you finally give up.....
Yes, I'm obviously being sarcastic, but for every 100 GT's that someone tries to put a 350 into, only about 5 actually ever see the road, the rest end up as someone else's project or in a junkyard. Be prepared for a LOT of custom fabrication, and no technical support. You're basically on your own.
You could build a tube chassis with a Mustang II front end, Ford 9' with a little engine compartment work on the gt then weld the gt to the tube chassis.
Another possibility is finding a Mustang II, Pinto, Vega, Monza or early 70's car of about same proportion; take the body, weld in new motor mounts for the 350, then custom metal fabrication work in the gt engine compartment and weld the body to the frame.
Pick something you can still get parts for, and good luck insuring it, or getting it to pass i/m and safety (depending on where you are). You'd have to insure and register it as a kit car.
By the time you're done you're really not much better off. It sounds fun but with the new Opel 2.4 and the five speed gitrag trans it would be just as much fun without destroying the unique or collectible value of the car. It would be a lot more worth the time and money to stick with the Opel parts.
There are still other options, V6's and bigger four cylinders.
I have a stock gt now and it runs and I really like it the way it is. I think if I really want more I'll go with the 2.4.
The 350 option is fun but by the time you're done you're probably looking at a track car only anyway. There's also electrical wiring work that would have to be done because the gt's harness won't hold up.
I had thought of putting a corvette 327 and a corvette 5 speed and drivetain under it; with the fender flares and the whole nine yards to make it look like a miniature vette. the best way I can think of to do that is to put it on some other existing car frame. You'd have to do sheet metal work because the motors we're talking wouldn't fit in an opel gt engine bay. If I remember correctly you'd have to remove part of the firewall and floor space.
That was one of my first ideas upon discovering Opel GTs.
Which ofcourse is likely why it is constantly asked on these forums by every new person who comes here. I read through a fair chunk of the forusm before bringing it up, which was lucky because all the information was already given out there.
Due to the 350's weight and size, if I were to have a second Opel GT (since I'd want one to drive around stock while I worked on a "performance" GT), I'd put in a Mazda Rotary. I think the 93 engine is the highest performance, however I can't pull myself away from whatever is newest and hottest, so I am sure I will get whatever super-modern rotary is out there.
I just want to scare the hell out of the person who is revving their Mustang next to me with the scream the engine makes at 10,000rpm.
Anyways, I am pretty sure I can get atleast 600hp on tap out of one. I worked on one with a friend of mine for a drag Rx-7. He was getting over 750hp and running under 10 seconds, mostly from turbos. It however appeared that he was able to turn down the turbos and have a pretty stable road-car.
You know, that's not really even a bad price for replacement parts. I wonder how the fit/ finish are and if they would work well on a street car. No more rusty fenders or battery boxes, and the crowds always did love the one piece front-end!