Hi,
just trying a shot at this one... I'm building a motor at the moment, pure street motor with lots of torque down low for a heavy vehicle. Doesn't sound much like what you're planning, but still some things are going to be the same, the most important is probably the cam.
For blocks any old block in good condition will do. Preferably 4 bolt, but even that's not a must (but 4-bolts are cheap enough nowerdays anyway). Build a 350 and you'll find the parts more easily for a lower price, build a 400 and the parts will be harder to find (if you go second hand on crank etc) but you have more displacement, that makes it your choice, and don't forget to drill steem holes in your heads if you're going to build a 400... What might be the best bang for the buck and is readily available everywhere is a Stroker-kit to build a 383, which gives you the best of both worlds, but normally requires clearancing and more block work.
Go hyper on the pistons, forged are only for race use, not for street. Roller valve train would be fine, but flat tappets will do the job just as well, if you're planning on mainly street performance. Stiff light valvetrain is always a good idea (and expensive). The 200cc heads look a bit big for that motor, because the port velocity might be too low, but as the Dart heads are very good out of the box you can still expect more than stock Torque low down. And good high-RPM capability anyway.
Now for the cam: I would send your specs (not only engine, but also car weight, intended use etc...) to as many cam-grinders as you can find and ask them for some advice on what cam you want to run. The more info you collect the better. Then buy at least cam and springs as a kit, as the springs will be taylored to fit the cam's range. You want the lowest pressure possible on the springs before valve bounce, float or loss of control. The springs you get with a cam will in 99% of cases be your best bet. And the diameter isn't all you should go by: the actual material is what defines how large the spring must be to accomodate the stress. Springs made of H11 tool steel are often smaller diameters and lighter than cheap steel ones. I'm just trying to find a cam for my engine and it certainly isn't easy, but the more info you collect, the better. The Dart heads are in any case a good start towards a tirefryer with nice HP
Good luck with your engine,
Max

PS: "Never use anything you already have only because you already have it. An engine can only be as good as it's weakest part, so do it right now or again very soon." David Vizard