Wow - I did not realise the propane lockers were optional - they look built in! I think a retrofit would be very challenging - mine are on port and starboard side, either side of wheel - you probably see them from the line drawings (e.g.
http://www.pearson323.com/images/drawing.gif). The lockers are substantial fiberglass "buckets" containing what appears to be a custom propane tank. They have a vent in the bottom exiting the transom, and a, inset solid hinged lid, closing flush, which matches the nonskid on the lazarette covers.
I can provide photos/measurements if required, but considering the amount of cutting/fabrication required, I think retrofitting to the original position would be challenging. IMHO Now or Never!'s solution would be the simplest and least expensive to install.