This is from the Burtz engine assembly manual.
We recommend that "single lock" Colony CM102-8 tappets be used for flathead applications. For OHV applications where the adjustment is at the rocker arms, stock Model A or Model B tappets can be used.
This link is to Marco's site,RIP, and shows how to shim a Fordor.
https://web.archive.org/web/20140819105055/http://www.abarnyard.com/workshop/door-1.htm
I'm not sure how accurate this information is but Les Andrews shows the 5/16" original thread was 5/16-32 and replacement tubes are 5/16-28 the 3/8" tube original thread was 3/8-32 the replacement is 1/8-27 pipe thread.