If you have the correct starter it should not require shims. Sometimes rebuilders install the incorrect nose and they don't fit right. Check the teeth on the ring gear as they could be damaged and be contributing to the problem. Also some headers cause starters to overheat, you may need to install some kind of insulation material if this is the problem.
Fred


1956 3100 Pickup/Red/350/3sp OD/PS/Disc Brakes
1957 Bel Air Sport Coupe/Red/355/TH350/PS/Disc Brakes
2017 Silverado LT Single Cab SB/Black/5.3/6 Speed Trans
1947 Willys CJ2A w/F-Head engine