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

If you don't care about gas mileage, the absolute best thing you can do for a streeter is to gear it up.
Going from 2.76s(typical) to 3.73s will make the engine feel 3.73/2.76 =plus 35% bigger;especially in first gear. Plus 35% is 79 cubes. So the ol' 225 will feel like a 304.
Add a 2400TC and she will be a whole new animal.At this point the S/Six stuff and a free-flowing exhaust will really fill things out.
Unfortunately, 65mpg is about 3000rpm, so this is not hi-way friendly.
But I can't over-stress the change in personality,that this big a gearchange makes. And it's cheap.
And you will want at least 3.55s with a teener too. And a 2400 to 2800TC, so it's not money you will never recover.
Whereas any money you spend on the slanty,is lost when you decide it is still not powerful enough. I highly recommend at least 3.55s; for either engine.