I was briefly pondering using a twin scroll Holset HX40, possibly a little on the large side, although a 16V head does move air very nicely. I happened to find a somewhat rare version used on some large Volvo diesel thing with a nice cast Volvo logo on the compressor housing:


It's twin scroll internal wastegate. Supposedly a T4 flange (I have the manifold already, it's non-external WG, T4). But when I got it in hand, it didn't have a true T4 flange, but a Holset variation - same width, .25" longer. Plus the WG was only on one scroll, didn't think that would work well on a gas motor - either to control boost or to keep both pairs of cylinders breathing similarly.

So I've decided to get something non-junkyard. A brand new gee whiz BorgWarner EFR 7064. It has a T4 divided housing with an internal wastegate that bleeds both sides, and is designed to control boost on gas engines. Very light wheels, ceramic ball bearings, should spool very well for the flow it's capable of. Just a bit of sticker shock...
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'63 PV544 rat rod, '93 Classic #1141 245 (now w/16V turbo)