Yes don't do it........Unless you are higly experienced and know what kind of work it invloves.
You must as said earlier build a tube frame and place the gt shell on top of it, but if you are going to build the frame you are better off using a fiberglass body anyway.....there is one on ebay right now!
The problem is the V8 is just far to heavy for the GT's frame, and withought a custom built frame with custom suspension it will drive like a brick.
For those that have been around opels for a while know all to well how many GT's have been hacked up because people wanted to put a V8 in them but didn't realize all the work involved and gave up. In fact for as many people I know that have tried I have only seen it compleated once.
But the good news is that there are other alternitives.
You can use the 3.4L out of the late model camaro with minor modifications, and someone correct me if I am wrong but it is actually lighter than the opel motor.
What I have always wanted to do was use a Rotary motor out of an RX7, they are much lighter, make almost twice the power(and can be easily modified) and are smaller so they fit nicely in the engine compartment.
Or you can buy a larger late model opel motor that would bolt in, a 2.2L or a 2.4L, or with a little fabrication a 2.0L 16V opel motor.
HTH
Chris Blust