The critical thing will be your chain line. The 170 hub width combined with your 164 BB axle width should do it. Not familiar with the rear tire, so I don't have much to compare with, but if it's less than 4.5" or so wide, you should be able to make it work.
The short cage rear der will handle your reasonably tight 11-25 cog spacing, and make for snappier shifting.
** Edit** I just saw the 20 x 4 Vee Apache that should work fine for a good chain line.
And, WOW is that CooL!
You might want to consider a larger chain ring, like maybe a 42 t, to get the benefit of that gear range. Otherwise it is going to be geared pretty low. And remember, you can always stand up and crank for short steep climbs.
Speaking of cranks, I would also consider a longer crank arm length. 140 is fine for single speed 'kids leg length', but if you want to ride this beautiful build, I would go longer.
Otherwise TJ, you definitely have my attention on this one! As always.... RaT oN~!
That big heavy wheel will be a lot to spin. You may want an easier gear than your Fastback, as the same ratio will be more work. The Worksman trike I roll at work is a ton, but its lower gearing really works for accelerating.
Edit: oops, I was thinking about another bike, forgot you are using 10x. You'll have plenty of gears!
Sand is commonly used when bending freestyle with no mandrel. I use salt because it's easier to manipulate (pretty easy to pour down the tube from the container) and it's cheap.
Where are the curved mandrels supplied with that bender,
and why aren't you using them?