Putting on 340 or 360 heads will drop the compression quite a bit. If you can find some 302 casting heads or swap over to magnum heads those are closed chamber and will keep compression up. Either that or do a 3 angle valve job with some pocket porting and gasket matching on the stock heads and put them back on.
318 is a good engine except for the pistons sitting down in the hole. Swap in pistons to raise that compression up, put on 360 heads, and a 318 will run like a 340. But that might not be in your budget.
By the way a replacement 340 cam would not be a bad way to go. Next step up would be the Comp Cams 268 that is a step up over the stock 340 cam.