[68gmsee]

Welcome to the site. Can you post more info on the truck like engine, carb, HEI or points, automatic or manual?

There's a lot of things that can make the engine run rough so check items mentioned above and I would recommend making a list of what needs to be done then check them off as you do them. Do the cheapest easiest first.

The leaking between the head and intake is probably a gasket problem. If it's bad, I would take care of that first as it may be getting into the cylinders. May also solve the running rough?

List of things to check and/or replace (some already mentioned).

Fuel contamination (filters, water in gas, old gas, etc.)
Vacuum leaks from old rubber hoses and gaskets
Timing
Ignition components (dist. cap, rotor, points, wires, plugs)
Vacuum advance not working
Debris in the carb

There are others but this gives you an idea.