Need opinions

I recently bought a 1991 360 out of a 3/4 ton pickup. Only 51,000 miles and I listened to it run before buying. Seemed to purr!!! Now the question. What could I expect for horsepower with a cast iron 340 intake and a 800cfm thermo-quad installed and not changing the cam? I need a lower rpm high torque output because of my highway gears (2.96 SG, 8 1/4) and tall tires. Also this is a 1970 Swinger that is used mostly on the highway but I want some go power between stop lights. I'm used to a 318 with a 4 bbl. intake and the TQ. I do have dual exhaust, 904 trans., factory AC. Second question. If I had a cam custom ground for my setup how much HP would I gain? I'm just looking for some round numbers. I'm still from the old school thought of " if it ain't broke don't fix it!" I'm weighing all my options before I do this "winter project". Thank you in advance for any info that might help me.