I was originally gonna go with a GT3582R, now looking at a GTX3076R. Should be instant, with a good PR capability to keep it from surging before the GTX4202R comes on. It'll be with a Steed Speed manifold Mike has for me, as well as an external gate.
I'm @ 7000 ft & currently running a T-4 manifold twin setup. Been toying with the idea of going smaller & considering the GTX3076r over GTX4202r. What HP range you realistically expecting to be at with that setup?