727 slipping in reverse when hot

Kim,
My initial thought was a front clutch issue til I read he’s having no issue with 3rd gear. However, anything is possible ‍♂️ be curious to find out the end result.
I had no issue with high get also way back then but I knew it had to be something. Wasn’t the vb either cause I had changed it from my other car and my other car worked fine with the vb from the tranny with no reverse. That’s why I mentioned the front drum. I have been wrong b4. There is only so many things it can be. Kim