The Errata page in the Internal Links section (WordPress Page) is an example of a Page where I used a Php include and simply pointed to my existing shtml page. It ends up ignoring some of the shtml includes in that page, and loses some formatting, but is reasonably readable. It required added the Exec-PHP plugin
The normal Errata page can be seen by following the Errata link in the Pages section.
I also set up a basically blank WordPress page for Contacts in the Internal Links section then added various types of include statements to the SHTML page to try and get the WordPress page to show after the regular page. No luck yet.
The normal solution to this is to add a line like this:
AddType application/x-httpd-php .html .php .shtml
to the .htaccess file, but when I do that then try and load my site I end up with an Internal Server error. Waiting to hear back from the Support guys at my ISP to see if they have another solution for me.