Nice job on the wheels! I did the same thing on my Coupe, but I used 15x7 wheels with ET25. The coupe is wider and I'm not sure they would clear the fenders on a Fintail, though it would be quite easy to try.
When I do the adjustment, I check the distance between the tire and the chassis rail behind the wheel, at 9 o'clock when facing the left rear wheel and 3 o'clock for the right rear wheel. You need to make the measurement when the car is sitting on its wheels in the normal position, not when it is jacked up.
If you end up with problems with the tires rubbing when you corner hard, let me know, there are ways to stiffen the system.
Could you post pictures of that fintail with the wheels and a hubcap (or at least one of them) so that I have an idea on the way it looks? I would also be interested in knowing the clearance from inner and outer fenders. A good test is to put one rear wheel on a high spot and turn the steering to check if the top of the front wheels clear the top of the wheel arch. I had surprises on my Pagoda and I had to review my set-up.