Back when I was considering a tunnelram for my 572 hemi, all I kept hearing was how small the plenums were. All the old Weiands were designed for near stock cube hemis, so they are on the small side for a 500+ inch motor.
As proven by quuickd100, they still make great power and a boatload of torque, but I bet his big hemi would make a bunch more power w/ a bigger manifold. I'd love to see him try something bigger b/c his motor is already a monster as it sits! Adding spacers should help, but it's gonna be awfully tall.

The Indy is the best cast hemi tunnelram, but even it will benefit from the plenum spacer that Indy offers on a 500+ cube motor.

I've played w/ all the big single 4500 manifolds on mine, but never tried a tunnelram. I had a 4500 flange Weiand for a while, but never bolted it on.