I didn't actually read your post till just now. He want's 800$ for that? Right off the bat, if it sounds too good to be true, it is.
If those rocker arms where quality brand, you're looking at between 250-400$ just for those alone.
lets break down what that engine needs, from the back forward
First, a flexplate. IF you go cheap on that, you'll pay for it when you have to drop the trans again just to replace it. You can land a good one for 100-150& SFi approved and made of good steel.
second, headers. It's pointless to even try and use that ONE header, right off the bat whos to say it will even fit in a gbody? second unless you want to miss match you'll have to find a match and that'll be hard. Luckily you can land a pair of headers for whatever price you want, used headers aren't bad, they don't have to be new. sh*t i reused a pair, but i had mine blasted down and powder coated, still only 100$ deep for a nice set of hooker headers.
Moving forward
Third, carb & distributor, that's money man, don't go cheap. Doesn't mean you have to buy the bleeding edge though, you can pickup a good used carb for a nice price or get one rebuilt, and the ignition system, dist plugs and wires is really whatever price you want to spend on them, unless you can find a used MSD or mallory setup.
fourth, waterneck and thermostat isn't sh*t, you'll be 30$ in for a good one.
fifth, All the forward accessory.
http://www.kmjent.com/cart/product.php? ... 542&page=1 That belt kit is amazing, especially for the price. I've bought a kit, run it on my car and it's never failed, never had any problems. And imitates a blower sound hah.
You can probably find a solid set of factory brackets for not bad price, and an stock power steering pump for a decent price.
Also checkout that websites waterpumps. They have a decent set of aluminum High velocity water pumps for Small blocks, I've been running one for awhile and closely monitoring the cooling, no problems so far.
Last but not least, your alternator. Just goto advnaced and pickup a CS style alternator for an 88 corvette. it should be about 100$, then go buy the correct adapter, i think this is the one
http://www.amazon.com/1969-1986-Corvett ... B000FLD1NM, i forgot where i bought mine, but the guys on here can help you find the diagram. They are really easy to wire up. So when it's said and done, you'll have upgraded to a newer alternator style, for a nice price.
And that should wrap up that engine, and you asked if a 355 will fit and bolt up in your car? a 355 IS a 350, bored over .030 to give that extra 5 CI of displacement. So think about it, it's a 350 block, why wouldn't it? unless your car previously had a V6, then you might have to swap springs and motor mounts to fit a small block into it.
Good luck with your project. IF you still can't find something you like then PM me, I have a long block, which is the entire assembled bottom end + cam and cylinder heads that i can sell you, it's a factory roller and with the right carb and ignition it won't disappoint.