First thing I did was pull all the front sheetmetal off John. Once all the sheetmetal is off you can then get a better idea of what needs to happen with what's left.
I took all the frontend pieces and laid out hole patterns to drill and bolt the individual hood, fender, grille supports, side panels, etc. together. Mine was a lot of pieces with about 8 pieces total to attach. Some of the existing holes from inner and outter fenders, and just drilled out to bolt to the hood.
With the entire front metal bolted together I was able to set it back on the car and then do some measuring and figure out where the best pivot point was to make it tilt smoothly and not hit anything. After I got the hinges made and attached to the frame and frontend, I then tilted it and found a point where it would go over the balance point, and then put the cables on to stop it.
Once I had it tilting and holding, I began adding bracing to make it structurally sound. The final step was to decide where to make the cut in the benders and then lay out a line on both sides. After the cut I drilled three holes in each lower fender and held them in place to mark the body where they would bolt on. I bolted them to the body and added another brace to them to make them ridgid. The 1/4 all thread rod made the lowers easy to match the shape to the rest the frontend.
I used a sawzall and jigsaw for all my cuts with a fine 24 tpi blade to keep it from grabbing while I cut. These made the cuts a lot easier than a plasma, as a plasma requires all the paint to be off everywhere you cut, and some areas that's almost impossible to access to remove paint.
On your Opel I'd guess you'll have similar issues that my Austin had. My frame isn't welded unibody, but it is bolted unibody, so I had a huge frame and firewall that extended into the engine compartment, and really took some head scratching to figure out how to eliminate. Here's a pic of mine before the SBC went in:
This what I had left after removing the entire firewall, and as I was mocking up the axle.
I ended up having to cut the entire top half of the frame off, and then boxing the top in. The outside of the frame was simply sheetmetal, so I had to cut it off and make the outside plate out of 3/16" steel. Then before boxing it I also built more internal bracing to strengthen it. In hindsight it would have been simpler to cut the frame off and stub out a new frame. I've probably got more time and metal in there than I'd have with a new box tubing frame.
This was after cutting the frame top off and welding the new top on. Test fitting the engine, then removing it again to cut the sides out of the frame and adding new bracing and extra metal to the outside of the frame.