Matt,
take a look at the 4.x PayflowPro package by Janine Sisk. This package includes an updated PayflowPro module based on the work by Jin Choi and Brad Duell. The module replaces cc_send_to_server_21 with a version for PayflowPro. It should not be too difficult to replace the original cc_send_to_server_21 with the PayflowPro version. Chances are that no further changes are required.
You'll find the 4.x PayflowPro package in the CVS development branch.
/Bart