When Champion obsoleted a spark plug, they were gone off of their replacecment charts unless a person can find older cross reference charts that list them and then go on to modern charts from there. Flathead Fords were limited to short reach and flat gasket type bosses so that's why they were only used on small engines after that. Various other small engine plugs have replaced some pf the older flat boss gasket type plugs. A lot of them are 3/4" or other smaller sizes now where the old ones were 13/16. Suffice it to say that the modern 860 spark plug is not the same as the old obsoleted 860 plug of the 50s.
Temporary flea-pay link to older long body 860 spark plug:
https://www.ebay.com/itm/14492659158...Bk9SR4jJgazGYQ
Modern short body 860 also described as a diesel glow plug:
https://www.ebay.com/itm/26610494970...epid=193845518
The tapered boss plugs were new in the Overhead valve era since there was less worry about reach with them but then the core electrode placements started changing to get the spark in a more advantageous spot in the combustion chamber. This opened up a million new plug types