I wanted to provide a follow-up on my original post. As I mentioned, I had inadvertently failed to install the refurbished 6 volt starter when doing the restoration over a number of years. The original starter was reinstalled on the rebuilt engine. Since I didn't realize that I had not replaced the starter, I was considering alternative causes for the extremely slow engine turn over when the engine was up to normal operating temperature. We'll today I replaced the existing starter with the rebuilt one and the problem is resolved. The engine actually turns over much faster when cold and just as quickly when fully warmed up. So, as was suggested, heat soak was not the culprit, a worn starter and a faulty memory were. Thanks to everyone for their help and suggestions.