Lets have some fun!

Yeah, the Performer does that, its good for stock 318 but 340 dies with it. If the engine really has stock-like pistons and is in good condition you should get close to 400hp (from the flywheel at least) with valvejob, cam and intake. Check the casting numbers from the heads. Ignition needs update too, MSD 6AL and a blaster coil would be good bang for the buck. And of course it needs tuning. Intake costs 235$, pair of edelbrock rpm heads would be little under 1500$...