I got a question I'd like to throw out that sounds novice but oh well. From a financial standpoint, if you could get a longblock L that will support 650hp...and we all know the quality of sunbelt's work....wouldn't this be an easier way to 500-600hp than an RB, 2J, or LS? Especially since everyone says that a stock RB will run an easy 15-20k to get running good, and manual trannies will run about the same cost to hold that kind of power.
All you would need is a Kick @$$ turbo and a custom header, and viola. No having to worry about custom this labor and fabrication costs, and your car being down for 2 years while you try and figure things out. And of course you still get the beautiful sound of a straight 6 L motor.