NAPA can get you a quality reman starter in 24 hours for about $70 with your core.
Yes, the upper fastener is a stud with coarse threads for the block and 1/2" fine threads for the starter. 1/2" coarse threaded bolt for the bottom fastener. Make sure it is not too long and bottom out the threads. I think 1" is the length. Maybe 1-1/4.
Yes, I would permatex that stud.

~~ Jethro
1954 3100
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1951 3100 (gone) / 1956 4400 (still in the neighborhood) / 1957 6400 with dump body (retired) / 1959 3100 panel (in the woods junked) / 1978 Custom Deluxe K10 / 1993 S-10 4.3 / 2004 Chevy Crew Cab / 1945 John Deere H / 1952 John Deere B / 1966 John Deere 2510 / 1967 John Deere 1020