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Use as much diameter and wall as you can stand, if the heads need clearanced then so be it, but if you won't do that then -

Double tapered 5/16"-11/32"-5/16" .120" wall and 5/16"-3/8"-5/16" .145" wall are available.

Wrap some tape on an old straight pushrod and mock up in the long block, keep adding tape until you notice interference, then back off on diameter about .020"-.030" or so at that interference point.



It has nothing to do with the holes in the head but the height of the lifter boss because I have a factory roller block.

Is double tapered stronger than no taper?



If talking like mid diameters the taper does not add strength, it only adds clearance.

Pushrods typically want to flex in the middle, that's where the column diameter and wall is needed most.

NASCAR guys use 9/16" pushrods with valve springs lighter than yours on opening and closing load. That should tell you all you need to know about how much pushrod strength means.