Posting a 15 second youtube clip and getting an internet diagnosis on an engine is about like going to WebMD and typing in "rash". You'll get everything from psoriasis to skin cancer.
What someone hears as piston slap may be nothing more than crap audio from exhaust pulses bouncing around your shop. How tight is the garage spot, what are the walls made of, how close is the exhaust to a wall? Don't answer any of that.
1000 miles on an engine built by someone with IMMs reputation should be barely broken in, not broken. More likely you're having some issues from letting it sit too long. White exhaust could just be condensation cooking off the inside of the pipes, a little smoke in the trans tunnel could be oil or ATF drips cooking off the outside of a pipe. The grey spots just look like a rich idle to me, same thing my car did all the time until I got it tuned right, and that same condensation inside the pipes will carry carbon out of the pipes along with it. I'll bet if you start it up and watch in a couple of minutes you'll actually have little black puddles underneath the pipes that dry up eventually and leave the carbon that was floating in it as dust. Valve train noise could just be that the lifters bled down and need some time with oil pressure to pump back up.
As Cab_Burge said just cranking it every couple of months and letting it idle isn't good for a car. If your carb doesn't have a four corner idle and you are only warming it up, then you could have had a bad gas situation in the secondary side gumming up the jets or needle and seat, since no new gas is flowing through. Happened to me personally doing the just idle warm ups over the course of 6 months or so.
You have good oil pressure and you said it drove fine the last time you took it out. If it was mine, first thing I would do is take it out for a longer drive to get fluids circulating everywhere and get the fluids hot enough to burn off any condensation. If that doesn't clear everything up then its time to start taking things apart. Check plugs for fouling, check oil for metal flecks, pull the carb bowls to look for crud, pop the valve covers and check lash/preload.
