Thanks Pilot DAR.
I had already found the type certificate on the FAA web site and it simply states that my aircraft must use a 76EM8S5 prop and it must be exactly 76" in diameter. It also specifies the maximum and minimum RPM in ISA conditions but makes no mention of what the pitch must be. The inference therefore is that so long as the new prop complies with the rev limits it can be either 60 or 62" pitch. However I find that strange since all the performance graphs in the operating handbook would no longer be accurate.