Jay, in keeping with the "There's no replacement for displacement" theory, keep an eye peeled for a 52-53 Chrysler 265 cubic inch flathead six. It requires a little massaging to fit it into the Plymouth, but most people won't be able to tell the difference, and the ones who recognize the swap will understand what you're doing and smile. The old Hudson flatheads gave the Oldsmobiles and Cadillacs a run for their money back when the forerunner to NASCAR was sponsored by AAA. Have fun!
Jerry