You are better off making use of the 318 heads even if it they are left stock. A light milling for a little more compression is a inexpensive mod that will help with a slightly larger cam. While a few upgrades to the head would be good for power, IE; good valve job, 1.88 / 1.60 valve size upgrade and then Bowl work in that order but not so absolute as at once...
Being it is a truck AKA Heavy, you can best apply torque increasing measures rather then seek HP. Your on the street trying to move a heavy thing so a decent head with good port velocity is important. You'll loose it a bit with the larger ports and valves of the 360 head.

As mentioned earlier, the price of the roller cam is pricey but there are a little nicer. You'll have to justify the expense.

After that, like the crew said, a dual plane, 600 CFM carb, headers and a upgraded ignition will enable you to go far with even modest cam sizes.