when do you run into this problem on a stock bottom end? Im just trying to figure out how big of a cam i can get away with stock heads and with twisted wedge heads.
Stick with the stock valve size (1.72/1.46) and it should allow you more than enough piston to valve clearance for a cam that flow's enough air for a street grind, without overwhelming the stock heads. You may even be able to use slightly larger (1.84/1.54) valves.
Don't be too concerned with getting a big cam, but more with getting the right cam. Check out a couple of the dual pattern cams that CompCams has available. They're a far better street grind than any of the alphabet cams!
It is not just lift that you need to account for. When the lobe is at it's highest lift, the piston is way down in the cylinder. It is basically the duration that you need to worry about. I believe it is between 30 degrees BTDC and 30 ATDC. Look at this site: http://www.geocities.com/jjonibones/PVC.html It has a lot of info on it, on how to check p/v clearance, when you would have problems, and so on.
Rick - I believe that it varies depending on the cam and the cam timing. On my set up below when I had the E cam, my closest clearances occurred at TDC on the exhaust valve, and at 5 ATDC on the intake. In any event, 5.0GT nailed it -- if you make a 'map' of the clearances from about 30 degrees on either side of tdc (during overlap) in 5 degree increments, you'll see where the closest point is.
SS - you do need to measure no matter what someone else's combo did if you want to be certain. Best thing you can do is work with a reputable vendor that will take a cam back (that hasn't been run) and allow you to buy a different one - assuming you cut it too close, and you don't want to cut the pistons. When it looked like my original E cam was gonna be too close, Central Coast Mustang actually sent me out a milder Crane grind - and allowed me to send back the one that I ended up not using. All I paid was the shipping -- pretty good customer service I thought.
Ultimately I ended up with a custom - I'm with Gearbanger; focus on getting the right cam for what you're trying to accomplish - not the 'biggest'.