One of my last handoffs from old Virginia Arrington to new Michigan Arrington (Diversified Creations) is this forum. I will point out this thread -- they might have ideas to help. They don't do email tunes anymore, though.
Carb Connection has been a reliable dealer for Arrington for a long time. This wasn't their first Arrington engine rodeo, although often they used Arrington tunes. I would guess that they simply wanted to play it safe. You are ~70 HP over stock for the 6.1 engine. But if you find someone who can safely tune it more aggressively (or talk CarbConn into it), you would certainly get more with the long tube headers.
As for block upgrades -- if I was taking your call a year ago, here is what I would have offered (and Arrington today will probably offer --
sales@shophemi.com).
Arrington has years of success taking the 6.1 block up to 440 c.i. This configuration is usually built high-compression and Arrington refused to put boost on it -- the walls get too thin. The most popular, though, is the forged 426 c.i. (7-liter) built with low compression (9:1) and set up for boost. I personally own one of these in my Challenger. Your block could be rebuilt to this spec. I don't think just changing the head gaskets is a great plan -- but it certainly would lower the compression. In any event mild boost (8 psi) won't hurt it just as is.
BTW, rebuilding it to a 426 is pretty similar to buying a new 6.4-based 426 stroker or just getting a Hellcat block. Do you have any other use for the 6.1-based 392? Shame to tear it down if it doesn't have many miles.
In summary -- should be more room in the tune (if done carefully), 8 psi supercharger boosting is fine (need a new tune, though), can rebuild the block bigger, don't bother with changing head gaskets.
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