In my opinion your cam, compression ratio along with rear gears and stall dictate what mainfold you want, big breathing engines with alot of compression and large cam profiles love single plane intakes.
Since you didnt give any cam spec or compression numbers I'll assume your running pump gas and don't really have alot of comp which would mean your probably right where you want to be with that performer rpm.
The heads you have are some pretty deep breathers though which is probably killing your low end torque some already - if you go with a race type manifold like the single plane Vic JR you'll most likely have a dog off the line but then again thats - JMO>Cp