68 truck 318 cam. How big can i go?

That's not a low cr build they squeeze 10:1 out of it.
If you use that as the basis for your build, you'll need more cam, cr and head flow to gain 50 more horsepower.

Those 400hp 318 recipes that float around would be good for a light deep geared A body but wouldn't be very fun in a heavy under geared truck.