So, Tom... Have you come up with a name for the Twin treatment?
I have a few ideas for a name.
"Bonneville" heads, based on the LSR goal for the build.
"Rock N Roller" heads, based on the roller rocker set-up.
"Pair of Aces" heads, based on the twin format.
"Ace/Mondello GP" heads, like we call our single versions of this layout.
Anyway, regarding your exhaust system, I did a few calculations, and it looks like you want to have primary pipe lengths of 30" each, measured from the back of the exhaust valve, to the end of the pipe, so you have to add whatever that exhaust port length is to the length of the pipe, and that would be your 30".
And the I.D. of the primary header pipes should be 1.5" because you are supercharging, and that needs extra room in the exhaust. That 1.5" figure is based on 150% volumetric efficiency, which is about what I figure that boost will be giving it. For N/A application the 1.375" I.D. would work fine.
And some other calcs I did on the intake side show it should support up to 8250 rpm with this port size in N/A format, and should be able to do even more with boost, if more is wanted. That's based on the 3.66" stroke crank from the Interceptor, because I think that's what you'll be using, right? If you use the 3.54" stroke, it will support revs even higher than that.