No, the rocker boxes will not bolt up to a Unit head 9 bolt or 10 bolt.
You probably don't need to use a 9 bolt top end either. You can sleeve the bolt holes on your Pre-Unit head or if they are just fine cracks as they usually are, run it like is.
Ace Classics in London, England has featured some articles in the past and seem to be in favor of sleeving those holes and I believe they make them too.
Originally Posted by latte hardtail
He also told me to look at the condition of the head as far as hairline cracks from the bolt holes inward towards the valves...... sure enough I found cracks. He said all of the 8 bolt heads end up developing cracks eventually; so I am in the process of gathering parts to change over to the 9 bolt later model cylinders and head.
On that note do the Pre-Unit rocker boxes mate up to later model heads at all?