As far as I know, no one has done this swap yet, at least not in the US. So this is essentially uncharted territory. I think you will be on your own to fit it.
If you are serious about trying this, I suggest you first try to find a donor vehicle or at least a vehicle you can measure the Ecotec engine from. I don't know if the engine is too tall, if the oil pan is an issue, if you will need to make a custom header (exhaust will be on the driver's side under the hood), or if you will need to cut the heater box to clear the FI intake manifold, etc.....
Like I said, you will have to determine all these things yourself.
Another issue is the bellhousing, the pilot bearing, the flywheel/clutch, the transmission type to use, and the driveshaft. Expect to have to upgrade the cooling system, as the stock Ecotec makes double the HP of a late model Opel GT, plus being an injected emissions-compliant engine, it runs at significantly higher temps than our old Opels. You'll need to replace all the fuel lines to accept high pressure, install a high pressure fuel pump, wire the ECU into the car, make a custom exhaust....
You have a lot of planning to do to make it work. Please let us know how it works out, I've often considered using an Ecotec in an older Opel. Good luck.
Bob