Well, the best thing he can do is tell Jim what he wants (just a general idea), what it's going in, what he wants to do with it, and how much he has to spend. Be realistic with both your HP expectations and your budget. Jim will likely either tell him how much he wants up front, and when to come get it, or tell him to take a hike (you never know exactly which one it'll be, depends on how you come across really). If he takes the job, give him the down payment and leave him alone until he calls to tell you it's done. Do that, and pay your bill when you pick up the motor, and you'll get along with him famously. (well, that, and listen to what he says about carb tuning, ignition timing, and first time start-up precautions)
What you will get will not be the same thing as a "crate motor" from Summit Racing or Oreillys. It will be a motor built just for you. Put a carb on it (with the CFM rating he suggests), a good fuel pump, a good distributor (timing like you're told), a good set of headers, prep it like you're supposed to before the first start, and you'll be a happy camper. There is no warranty, no guaranteed mileage, no guaranteed HP rating unless you have him dyno it first. If you forget to fill the oil filter up and start it dry, you pay to have it fixed (and you'll likely get the stink eye too). That's just the way of things in the performance engine world.
If he'll build it, it'll be strong for the money. Just be aware that insane HP always costs insane $$, so unless he has a small fortune to burn, expect to end up settling for moderately ridiculuos HP for moderately ridiculuos $$.
Besides, unless you drive race cars for a living, 800 hp on the street is dangerous. And even if you can handle it, you'll only make it across town on a tank of fuel. Better to shoot for the middle and end up with something that's pretty easy to drive, and that you can still go for a (short) trip in. I'd say anything in the 350-450 hp range is very doable on a moderate budget, and you should be able to expect a decent life out of a motor like that.
I can't remember his number right off the top of my head, but you can go look in the Rolla MO white pages for Covico Manufacturing and get the number that way.
Jim's a hell of a guy once you get to know him, but he can be grumpy sometimes too. The less BS that is involved, the better people get along with him. Just be polite and concise in your requests, and you'll be fine. Be a jackass and he'll probably just hang up on you. lol