Is 3" exhaust to much for a 318??

While I agree that motor would be fine with 2.25 from my experience I have never noticed a loss of torque from running 3" pipes on a mild small block.

I went from 2.5" to 3" on a mild 340, low compression, headers and intake and carb. I didnt notice a loss of torque. I ran 2.5" on a 318 B body. Motor was stock except for the electronic ignition. stock 904 and converter, 276 gears. It had no problem spinning the tires.

I would keep what you have and maybe add a set of 2.5" tail pipes.