The cast iron headers should work nicely for a turbo app,as far as I know Tom Langdon said nobody has used them for a turbo app,but maybe for you guys that cannot use the SPA manifold,maybe you can use Tom Langdons manifold instead of fabbing a turbo manifold header.
I though about them,before I bought my SPA manifold.
Running a twin turbo set-up, will cost a lot more, complicate things more,more plumbing etc.
You can select the correct turbo for your application easier these days, but for the WOW factor you can use dual turbos,& get a little better streetabiliy especially if you want to make big power, running a larger single turbo will spool slower than 2 smaller turbos.
All depends on your goals.
MBHD
