Chuck,
Been through the starter-shimming exercise. It was one of the Nippondenso gear reduction units I bought as a "super starter" for the 460 that required a shim.
If it needs shimming and you don't catch it before you try it, there will be no doubt after you turn the key for the first time
The thing will shriek like you wouldn't believe. Don't remember exactly, but it took somewhere around 0.040" or so to get it corrected.
Folks will say Fords don't use shims. Not entirely so: some aftermarket starters for the 460 DO require them.
All that said, my favorite starter - which I am currently using and very happy with - is the stock 1993 Ford F250 460 starter. I used the 460ford.com guys' suggestion and bought one. It looks anemic and is lighter weight than the others I tried, but it starts my car hot, cold or in between, without fail - something all the others (I tried a total of four different ones prior to this one) did not do.
Tom