Hey Scott,
Cap screw from hardware or wherever you can get some.
You don't say, but I'm betting that you are making the part for the pushrod bottoms to sit in ?
I've talked to your head maker (Serr) last year and he did not have any available at that time. Serr and another such head maker said same / same...'modify cap screws'.
So, I talked to a talented machinist buddy here (rogerroadster) and he had just made up a batch for his original Cragar install. He used a 'ball mill' tool and cut the cap screw to fit the lower end of the push rod.
I have a set somewhere and if you think that a picture/measurement will help, let me know.
As for how long the threaded end is, if I remember there is a lot of threaded area into the lifter body to work with. From memory the cap screws were about 1/2 inch or longer thread area length. I may use mine to install an original Schofield...or maybe not
ursus....'how long is a stick', good one..ha ! Or how high is up ?