Others can give you an idea what their clearance was on a similar combo - if they've measured it. Most haven't. The problem is that a combo with identical components can have significantly different clearances. The only way you'll know your exact clearance is to measure with your cam.
There are some ways you can mitigate the uncertainty, but you'll have to take some measurements. First, you'd need to go ahead and get your heads, and pistons and do a mock up install to take some measurements - no way around that step. You'll need a degree wheel, and a dial indicator and a positive stop (to find actual tdc) - the same equipment you'll need to degree your cam or measure your p to v, so you're gonna need it eventually anyway. Basically, you'll need to remove two valve springs (int/exh for #1 cylinder), and using the degree wheel and dial indicator, measure how much distance there is between the valve being closed, and the valve touching the piston at 5degrees btdc, at tdc and at 5 degrees atdc. If you provide those measurements to any of the custom cam designers out there, for a given rocker ratio they can compare that to the lift at the valve at the same points (-5, 0, +5) -- the difference between that 'drop valve' measurement you made, and the lift of the valve at the same point is a pretty good approximation of your p to v clearance; so they can come up with a cam that should give you the clearance you need. You might be able to get the same info out of the tech lines at some of the cam companies -- but I wouldn't hold my breath. Most of the folks I've talked to are more marketing folks than technical specialists.
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