300 HP out of a 318, is it possible with bolt ons?

In a given engine, the best way to solve this is with a higher starting cylinder pressure. and Second best is slam that intake shut sooner. You need a fast rate of lift, Solid-Lifter cam, in a smaller than normal size, which I haven't seen in a long time.
i'd bet something from the isky catalog would be a nice choice (barring a custom grind). they have several that are conservative lift but decent rate without going overboard on duration.

something like that with the OE adjustable rockers could be a reasonably priced way to put a little pep in the step if you're trying to be low key and not break the bank.