I would imagine they all use the same keeper, so that shouldn't be an issue. Whether they're 7° or 10° I don't know, but they wouldn't change them between valves. Your best bet for measuring PTV clearance would be to pick up a dial indicator and measure against the rocker tip. I'm not sure how it would work in your method, because you need to remove the head and mic the clay on the piston to see how thick it is, and how much clearance you have. If you didn't rotate the engine over, the clay wouldn't be accurately compressed.
All you really have to do is two lifters max, enough for one cylinder.
I would probably only clay one cylinder unless the PTV clearance is really close then would think about doing the rest just to make sure.
http://www.stangnet.com/mustang-forums/758168-how-make-solid-lifter-checking-pushrod-length.html
BTW when I put my TFS stage 1 cam in I bolted it back together and never checked PTV clearance. Pretty close to the same combo that you are running with the P heads. Not saying its the right way to do it but more than likely you will not have a PTV clearance issue.
I would probably only clay one cylinder unless the PTV clearance is really close then would think about doing the rest just to make sure.
http://www.stangnet.com/mustang-forums/758168-how-make-solid-lifter-checking-pushrod-length.html
BTW when I put my TFS stage 1 cam in I bolted it back together and never checked PTV clearance. Pretty close to the same combo that you are running with the P heads. Not saying its the right way to do it but more than likely you will not have a PTV clearance issue.