The Indy 440-3X manifold you mentioned is a big volume manifold for big engines. Unless this engine is going in a real light car and/or is gonna turn a lot of RPM, stick w/ the smaller Indy 440-3. That "X" on the end of the part number makes a big difference. It's big so your lowend grunt is really gonna suffer unless you're set up for it.

I was told 440-3X was designed after Indy got upset seeing their big Indy heads being run w/ the huge B1 intake manifold on them, so they made their own equivalent.