Hi Tim
Not really at the power level your looking at as the std plenum is fairly good, with actually little flow loss. A custom plenum will end up a very similar result for a lot of effort.
A stage one type head and Piper 270 should make around 130-135 without to much trouble on an 1800 bottom end with a decent exhaust system...closer to 140hp is possible if the airbox is modded and a tubular type manifold is used.
Peak numbers are less important really for a road engine as you want good low-mid range grunt which the above should provide.