68 truck 318 cam. How big can i go?

Here is the skinny on milling to increase compression. The compression on these engines is lower than what they are rated for. By a good bit. Chances are your 318 will blueprint down in the high 7s. Probably 7.8 or so. Sad but true.

The reason is because Chrysler had terrible machining tolerances when it came to deck heights that were too tall and combustion chambers that were too large and pistons that sat way down in the hole at TDC.

Now, generally, it takes about .070" of an inch off a small block head to equal one compression ratio worth of change. That's a BIG cut. More than most machine shops can do in one pass.

Couple that with all the cuts they will have to take off the heads on the intake side and the labor will add up quickly. Milling heads is not a good way to increase compression. Can you do it? Yes. But plan on buying custom pushrods and possibly having rocker arm geometry woes as well. You could possibly even run into piston to valve clearance issues.

You have members giving you advice based on their DECADES of experience. I'd listen if I were you. We are trying to tell you the best way.

By running a thin head gasket and maybe taking .025 off the heads max, you can get to around 8.5:1 which is perfect for crappy pump gas on something you will drive everyday.

That cam I recommended will act like a Thumpr or Whiplash. It will sound bigger than it is because the DEH grinds have long exhaust duration and a big duration split just like the Thumpr and Whiplash......yet they will deliver more power down low, because that's what they are designed to do.

My advice is shoot for 8.5 compression. You might not be able to quite get there depending on how low the compression is now, but you can make a marked improvement and have a snappy street motor with smart parts choices.

Lastly, if you are just bound to have compression around 9:1 or greater, do it right and buy pistons. But you honestly don't need that on the street everyday to have fun.

You've been given good advice. You should use it.