Ran the .509 for years....and I can tell you that there is NO way a 2400 rpm convertor is a choice at all!!
3000 - 3400 is what you need. And if you're worried about not being able to "cruise" you're worrying too much!!!
My Challenger was a tad "loose" under 40 MPH with a 3800 B&M Holeshot, but on the highway it was no problem. No difference in mileage to speak of.
Don't waste the time with a low stall convertor...get one that will make use of the cam.
That .509 is a great cam.