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

If staying with the 225, you will be happier starting with RRR advice, Super Six intake and 2 barrel carb. Use 2.25 exhaust with a low restriction muffler. If that is not enough for you then add a smallish cam, valves springs, and double roller timing chain. I would never bore a block that did not need it. While the head is off, replace your valve seals with Viton valve seals and find true Top Dead Center and mark it on your vibration damper. I don't guess Horsepower numbers, but this alone will give you more power and usually better mpg.