1974 318 duster. Can I port/polish these heads and get enough power out of them?

Step 1...... figure out your total budget.

Step 2...... determine how much the essentials are going to cost.

Then you can see how much is left for hot rod stuff.

After you figure out step 1........ see if that dollar amount realistically coincides with your goals.

Re: your 300hp goal......
30 or so years ago I re-ringed a previously rebuilt 318. I used mildly ported 318 open chamber heads, a std performer manifold, 340 cam, and small tube headers.
It made just under 300hp.