Just a general reply to a few different posters:
The 'Unicorn Corral' part is also somewhat to do with the likelihood of this bubbling up to the top of my projects queue any time soon...
that may change of course...
Yes, I'm looking at 1.5" primaries, it's the smallest size that matches the port area.
Great idea about using half of an ebay 4-2-1, that just might work.
Planning on fitting it by using a "ramshorn" layout at the exhaust valve - have yet to measure though.
Variable geometry is fascinating, but I'm at the limits of finding good design info on long 4-2-1s as is... might keep this one 'simple'
For fabrication - I love TIG welding, and mandrel bends aren't stupidly expensive (see
ECS engines for example). Also there's a local shop that does CNC mandrel bending for reasonable prices. Will see how I go.