Whatever you do combining the Golf1 and Golf4 4WD floorpan, as already said you're going to have to do a lot of work at the DVLA convincing them to give it a registration mark. What you're proposing will fail hard on the points system.
Syncros aren't 5-door only BTW.
Plenty of 3-door LHD ones around on the continent. LHD-RHD conversion will be a walk in the park in comparison to the rest of this little jaunt.
I would suggest that a Golf2 shell (especially a high-tunnel one) would put up with the power a lot better than a Golf1, especially a cabrio. For a start you're going to have to fabricate up mounts for the front subframe, which will be there on the G2 frame. You can also nail the Golf4 transmission under the Golf2 without completely cutting out and reinstating the floorpan. This is important because it means you'll avoid all the hassle with the DVLA, which is going to be a nightmare to get this on the road. I would suggest on a Golf1 you will only score a Q-plate. On a Golf2 frame you might be able to hold on to the original registration.
Hope your mate's got deep pockets, because what you're suggesting (or what I've put forward) is going to need them.
