One thing that's worked well for me far as headers go is to select a set of headers for the particular engine you're using in the smallest car the factory stuck that engine in.

Small example was an FE GT 390 into a 59 Ranchero.
Headers used were for an FE into a Mustang.
They were a good fit, the right side required no alterations, the left side required the front primary altered a bit and set up so it would bolt in by itself.
'Course, it was the last thing in on the left side, but it was easy.

Did the same thing with a different brand of headers when the FE was installed into a 62 Ford wagon.
The front left primary on that needed a little more work, but it was still easy to fit.

What's required is to saw the full length header flange in two between 1st and 2nd cylinders on the left side, saw the front primary in two in a convenient area, weld a two bolt diamond shaped - easily available at muffler shops - plate to top end of primary tube (down near collector) and another on bottom end of primary tube.
The three tube header is bolted in.
The single tube front primary is bolted in - with gasket - and then bolted to the front exhaust port.

Hope that's not too confusing.

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Bob, thanks for the pic of the "crab claw" manifold.
I keep hearing about these and have never seen one.
Looks like a good way out and I'd venture to say they would flow as good as a set of the cramped block huggers.