Next steps for more power - 318
Bigger valves and more compression. ...is there a good off the shelf set of heads you'd recommend?
To be honest at this level of performance they are far from holding you back. Get a quote for the heads to be re-valved and shaved a bit and you will then have a ballpark figure.
Aftermarket heads (like Edelbrock RPM's) may flow well but are also relatively costly, and your engine combination will not fully utilise them in its current form. Bang for buck would be getting your current set (or even a junkyard set of 318 heads so you can keep the car on the road) machined and re-valved.
Have a set of X Heads and 268 camshaft ready to go... I'm thinking about putting the heads and camshaft on my running 318. Would 350hp be reasonable?
Depends. It may make that number with a bit of compression and that camshaft but it may not be very streetable. The heads as far as I am aware are around 68cc chamber volume so will keep your comp ratio the same more or less, but flow a lot better.
The camshaft would be getting edgy for a well-modified motor with 10:1+ compression and a modern port-enhanced intake (eg Torker II, Performer RPM or AirGap).
Basically... you will have a screamer of an engine in current form but not a lot under 2500rpm. The raw numbers are harder to put a finger on, but with stockish 8.6:1 compression you would be lucky to see 300hp.
- boingk
EDIT: Here is a good read of a past HotRod Magazine article. They sure get a lot of power out of a 318:
Mopar's 318 Engine - Cheap as Dirt