Heli-coil questions

It should work. We did this sort of thing in the textile mill often. the cheap cast iron pumps they used to move liquid latex eventually has a bushing and stainless stud with a nut in every hole. The outer threads were 1/2-20 NF because the casting was less than 1/2" thick. Fine threads equals about twice the surface contact. Just something to consider next time. Anyway..
The plug was staked to the casting, final thread scored and such to prevent the bushing wanting to turn with the stud next time the pump needed a new seal.