Tunnel rams love rpms and are notorous for being a dog down low, just the nature of the beast.
Your set-up is begging for more compression and cam, but other than that your gonna have that low end bog, and getting bigger higher flowing heads will only make it worse.
You'd probably get into the mid sevens by just putting on a single plane intake and running a single carb like a 750 double pumper.
Goodluck, Charles