For that price I would get a crate motor, but to each their own. What would sway me is the 3-year warranty from GM on their crate motors, and I have seen awesome prices on their motors. I'm doubting that the guy building the engine would give you much warranty past the start up of the motor.
Thoughts on a few of your questions:
The Edelbrock air gap is a really good intake. It flows quite well, and helps to keep the engine cooler and limit heat into the carburetor.
The eDelbrock carb, however, I would scrap in favour of a 570 Holley Street Avenger. You'll be much happier with a street avenger.
Go with manifolds. Exhaust HP gain claims are always much higher than what you'll actually see on a dyno (and especially way more than what you'll feel when you jam on the gas), and headers only make a difference if you're racing (high rpm all the time). Stock manifolds flow plenty well for street applications.
I changed my 235 in my '62 Bel Air out for a 0.030" over 350, and the stock rad has no problem keeping the SBC cool. I'm not sure if the radiator is sized differently on your truck, but it should be fine.
Is the 320 HP he quotes on a dyno test? A lot of engine builders can say that the engine will be a certain horsepower, but that better be quoted with a given cam, lifters, rockers, intake, etc. , and you better have a dyno sheet to show that. Otherwise, you don't know what you're getting is actually 320HP or 290, and you won't know when you put it in your truck, as it will be a big improvement over the 235 so any hp level will seem like 320, if that makes sense.
