Well with mud and street use. I would think the smaller would be better. It's a lot of truck to get moving. And you're going to need torque for that.
If I was you. I would phone some custom cam grinders (racer brown, bullet, etc) and see what they tell you. A phone call is free. And a custom ground cam, built exactly for your setup doesn't cost anymore than an off the shelf. In fact I believe jim over at racer brown is cheaper than the comp options. I could be wrong on that.
But when you call. Have EVERY spec ready. You better know how many farts you drilled into the seat cause they might wanna know that. Take a look at this to get an idea:
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If you talk to those guys. They will really help you build a custom cam that will make the most out of your setup. And help you figure out how to get the most out of what you got. Honestly a 416, with heavy ported x heads, 10:1 compression, the right solid roller cam, proper exhaust and a good ignition/intake setup (dump the 650, get at least a 750 double pumper doesn't matter what brand), should make an easy 550/550 horse/torque. It will be a lot of fun no matter what it's in.