Norton factory pushrods would not work on (dunstall) BSA style lifters.
You would need a cup on BOTH ends.
The length will depend on the specific cam/lift profile that you intend to use. Some real dunstall 810 kits came with milled heads and reangled intakes. This also affected the length.
Pete, you are skating on thin ice using that 810 kit unless it has been re-sleeved and fitted with thicker sleeves. The stock sleeves were notorious for cracking and destroying your motor!!!