To answer a few lister questions:
No, it wasn't a kit, it sprang from the imagination of the PO who engineered
the manifold from scratch.
It's a '70s era "RAYJAY" turbo.
While I've made two different adaptor throats to fit bigger and bigger SUs
over the years, I've never made one to fit a Weber because the SU is great for
this application and with only one, there is no need to synch it to another,
eliminating the major SU hassle. With everything I've read on the list over
the years about tuning Webers to specific applications I realized I would be
in uncharted waters and would end up buying a butt-load of throats, jets and
emulsion tubes until it got right.