Max pipe diameter for a 318?

The manifolds on my '85 pickup and '88 van with a 360 would only take a 2.25" pipe so your manifolds are probably the same or they may only take 2". If that is the case I would do a step system form the 2" to 2.25" pipe. Exhaust is just like a camshaft and bigger is not always better and can be worse. The exhaust system in a normally aspirated engine (non-forced inducted) helps scavange the cylinders. If you go to big you can very easily loose torque and horsepower.