Aftermarket turbo exhaust manifold?

Well, I certainly wouldn't argue with expert consensus. Seems a little odd to me; O2 sensors are routinely subjected to much greater heat than it seems likely you'd have at that location with that engine, but I wouldn't want to be the one paying to clean up the results of being wrong on that point.

I share that concern about where the turbo's gonna wind up in that '64 Valiant.