Bobby,
I have a single '290 casting that looks like it's never had a valve job or been touched. There is a guide boss that needs replacing, from memory, due to being chipped on the valve spring side. If you find another single, and need just one, I kept this as a spare because the seats looked so good. Small plug too, I will check it to confirm the details and send photos if you need it.
Grant Eldridge
grant4060@yahoo.com