I've never played at that level, but 18.4-38 full cuts at 6500lb at 6mph is serious. I don't think 500 cubes will be enough to win.
Look at who you will be competing against. Another guy like you who asked the same question: what do I need to win? A 409, 421 or a 436 aint gonna cut it. The little tire 4mph guys all have those.
I would think that you need to be in the high 500's or near 600 cubes and over 150 HP. You are getting up with the very big boys with those tires and that speed.