I am thinking set the 12" box 6.5" from the tail and the rear plug at 17" as shown.
I want to be able to run it as a single or thruster.
The tail width is 13.5", one foot up.
Any thoughts on that set up welcome.
Well… don’t you love it when a plan works out? That board looks great.
On the fins, if you want an actual ‘thruster’ vs a ‘2+1’ you might want to install the center box as far back as tail thickness will allow and perhaps move the side fin placement back a bit. A small center fin with the screw tab forward will allow maximum rearward positioning in the box. With some whittling, you could remove some of the fin tab in the back and slide it back even further. (see photo below)
As a ‘2+1’ I’d say you’ve got it just about perfect with those fin placements. Position the side fins with very little if any toe-in. A set of longboard sidefins with a center fin of about 6" should do the trick.
for that 8’6" put the rear of fin box 4" from tail, plugs for thruster rail fins 12.75" from tail, 1.25" off rail, toe in 1/4" (should point approx 3" off nose on each side).
That’s assuming you want it to turn. If you just want to go straight, where you have them is good 
Thanks gents. You can’t see any of the blemishes at that distance John but pretty pleased with the result.
The next board will be white though!
Tail thickness was an issue an issue but I did go back 1/2" with the box.
The wave I have in mind for this board I am happy to go with a single.
My son wanted some fin options for other locations so will let him decide where the sides go.
I’d say go with Keith’s recommendations but if tail thickness limits how far back you can go with center box, the 2+1 is still a viable alternative.
I’ve seen some 2+1 sidebite setups with the sides pointing nearly dead ahead but the leading edge of the fins has been shaved on the inside so the leading edge is nearly 50/50. The inside of the foil is still flat. The sides can also be smaller than a conventional thruster setup.
I’ve done side bites from the tips cut off standard center fins… I.E. completely symmetrical foils. These were installed with slight toe-in and slight cant. This seemed to work fine on a 2+1 setup - with minor adjustments with the center fin I couldn’t really tell much difference between those and standard side bites.
The closest thing I could compare to would be an old school fish with symmetrically foiled twins set dead ahead with zero cant. It works fine once you get used to it.
Your leash dragging behind the board likely creates more of an effect.