How much extra HP can I realize from adding a cam and 4bbl to slant 6.

What I am finding with my own car, is that head porting makes a HUGE difference with these engines.
No dyno numbers to share, and I am still sorting the drive line, so I don't have any representative strip times just yet, but seat of the pants is amazing come 3500 rpm or so.
The head is one I cobbled together just as a starting point. Lots of time spent in the bowl area with just a clean up of the ports themselves and back faced stock valves.
Over the winter I plan on carving up a full race-treatment head. I'll post some pics of it as I move along.