I run an Engle KV-3 grind (252* @ .050 and .573 lift, square pattern) and like what it does. Jeff Patterson helped me with this choice. Best advice (as already said) is to call a couple of cam companies, and talk to them about what you want.

Your combo is basically the same as mine in an RB Block, so I can offer that mine is streetable, but offers good punch all the way to 6500.

FWIW: What I WOULDN'T do is build a motor as a stop gap. All you're doing is taking money from the build that you really want. If you want big cubes, you're only a crankshaft away with what you have now. You're into this motor at least 5k, and a Source arm is only another $800.... doesn't make much sense to me??? But, the other side is that 446 is plenty of cubes to make an E-body run strong. I guess what I'm saying is: whatever you build, build it right and don't think of it as a stop gap. We all want the Indy headed 512, but building the motor and driving the car are WAY more fun than dreaming and wishing. So be realistic about your budget and how long you're willing to be patient, and execute that plan.