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

Bolt-on's I don't know. Starting clean-sheet but on a budget, get '85-2002 replacement pistons (more compression height than the older pistons, even the 4-eyebrow early style) and start with #302 casting (heart shaped chambers) heads, modified for 360 valves (1.88x1.60"), with chambers deshrouded and bowls hogged/blended. Port match the intake ports to the pushrod pinch to 340/360 size and blend past the pinch. Mill the heads to reduce the volume back down from the deshrouding cuts. Pick a conservative camshaft, at least a 218*@.050" but probably more like 228*@.050" and at least .450" lift up to about .500". The last one I did, I used a 270S Comp Cams solid flat tappet with 273 rockers/pushrods, 224*@.050" with .468" gross lift. Very mild lope and plenty of RPM. I used a single plane intake, but your Performer intake opened-up to 340/360 size, or even a factory 340/360 iron intake would work. Headers are a must, 1-5/8" with 2.5 or 3" collectors. Use a muffler that is straight through with packing, avoid FlowBoys or any chambered muffler. Cherry Bomb Salute and Jones MaxFlow are cheap and sound great with better flow. I use a Flowmaster merge collector and a single 3" muffler/tailpipe to build torque, but 2.5" max duals work too, with a H or X pipe after the collectors. This setup will work on regular pump gas, if your budget allows, run SRP or KB zero deck flat tops and premium gas.