It'll help, but not enough to be worthwhile. If you intend to just do cruise-ins and drive the car around...go for it. I'm sure it'll drive good on the street.
Just my 2 cents, not trying to talk you out of it if that's what you want to do. I feel like your car would run better w/ a smaller manifold, but not THAT small.

No. Its a good discussion....We talk wedges so much on here, its nice to talk Hemis once in a while. You thinking the Barton or the -3? The only reason I never went with the Barton was really the non-removable center plate and the weight really. We had a 4150 version on a 484 Hemi, and it ran like a scolded monkey....It was basically a street car and went 10.26 carrying the wheels to 2nd gear...I figured the Barton would be too small for 600 cubes.

And I have run a tunnel ram on my previous Hemi, just not one this big......

I think the 4500 Barton manifold would run better on your combo. I'm assuming they make one w/ the correct bolt arrangement to fit your heads?
When I tested the Barton and the big -4 Indy on my car, the difference down low was surprising. The Indy -4 screamed up top, but was almost a full tenth slower to the 60'. With a looser converter and the ability to turn more RPM, it would run well NA. My combo just isn't suited for high RPM.
The only reason I currently have the big -4 on my car is b/c of the nitrous.

The smaller 426-3 matches the Barton's lowend power, but is really lacking up top. The only reason to use the 426-3 is if you have hood clearance issues b/c it is a good bit shorter.
The Barton is way heavier, but it makes up for it.

Overall height...............Weight.
Barton...7 & 9/16" .........35 lbs
Indy 426-3...6.5"............19 lbs plus 3 lbs for the valley plate
Indy 426-4...8.25"...........22 lbs plus 3 lbs for the valley plate

Pic is the Barton on the left, the Indy 426-4 on the right