Mike, count the freeze plugs on the side of the engine. If it has 5 it's definately a 200 not a 170. If there's only 3 freeze plugs it's probably a 170. You might have a bit of trouble seeing the back 2 plugs, so look closely.
As far as the 'BEST' engine year, you aren't going to find the 'BEST' setup in a standard longblock. To do that will require you to have it custom assembled. The early heads had a 52cc combustion chamber giving the engine a 9.5:1 compression ratio, but they had rather small and restrictive valves and intake runners. The later 'Big Log' heads (77 and later) had larger valves, a larger carb flange opening, and bigger intake runners. But they also suffered from a large 62cc combustion chamber which gave them a lower compression ratio. To get the best of both worlds, you can take a 77 or later head and have it milled down to get back to a 9.5:1 CR.
It really depends on how much effort/money you want to put into the project. There's a fairly wide range of performance upgrades that can be done to the 200's. If you plan on going this route, doing the longblock right is the best way. That way you will be set for later "Bolt On" mods like carb, exhaust and ignition systems.
The way many people here have done it is by aquiring a good rebuilable core, and having a local engine rebuilder build it up to your specs. For example, here's what I'm doing longblock-wise. I picked up a good 200 core out of a 1979 Fairmont. Ported and pollished the head and had it milled down to 52cc, and had new valves and springs installed. I took the shortblock to a local rebuilder to be remanufactured. Along with the standard remanufacturing work, I had the block decked to zero and had them install a 264 112* performance cam that I provided, along with a billet duel roller timing set and ARP main/rod bolts.
While I did spend more than I would have had I simply bought a reman'd longblock, it was only a couple hundred more, and I have a longblock that will be set for most any performance upgrades I may want to do.
