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Single plane manifold. Little low end, a bit of mid-range and a bunch of top end. Not a good street unit and in a truck engine is gonna be worse than a car motor.
Personaly,I like Offenhauser manifolds.I had a GTO with an offy 360 manifold, it ran strong all over.But then, thats another animal alltogether...But I developed a liking for the offy product.The 360 has a divided plenum so it will be more responsive to lower RPM's than an open plenum.I am currently running an offy dual-port manifold and like the low end get up with the plant your *** launch when the secondaries open. given the opportunity, I'd try the 360 in a hot second! Where is your foot when you drive? Mine goes down close to the floor,my 390 /66 likes to be driven hard.(me thinks that I digress a bit...) I like OFFY.
I don't know what the rest of your set up is but the dual plane you are running is a good bet for a truck manifold. If you feel like your missing some top end you might consider a switch, but the one you have is well designed for a low end and mid range engine.