I know the difference between the two engines. Location of the cam. What would the Hp difference be between the two? Supposing the cams have the same amount of lift, duration, and everything else is the same minus the cam in the heads.  

if the valves open, the valves open... ohc elininates the pushrods. there might be a reason power wise, but i see no diference in how the valves open. i have always wanted to build an OHC sbc head with a sort of Vtec system. the same way hondas work. one set of lobes for idle to 4000rpm, and another set of lobes for 4000rpm to 8000rpm. lots of bottom end with lots of top end. but thats another story...  

Probably the more significant gains come from adding another intake valve and drastically improving breathing - it changes the whole 'ballgame' when an engine isn't so severely undervalved as a parallel valve engine...

So the difference in power with identical ports and identical lobe profiles the only difference being an OHC is probably pretty slight to a relatively low rpm street machine.

The gains in power in terms of valvetrain stability would only be realized at extremely high rpm's or if the valvetrain had to be crutched in some area.