common dilema..
My 400 SB I had in my 74 K20 (from a 79 Suburban) was tough on starters too---I put at least 3 in it,and the previous owner of the burb said he put 2 or 3 in it too--when I bought the burb for parts,I had to rotate the engine by hand and try several times before I got it to start..flexplate was trashed in one spot bad..Chevy motors do seem to always stop at the same place 90% of the time,for some strange reason..--usually on #1 cylinder..
I installed a "good used" flexplate and starter I had,and it only lasted a few months..I had the starter fitted properly,and used no shims..it still sounded like it was too far away from the flywheel,and shimming just the outer bolt didn't help much..the nose cracked on the first starter shortly thereafter...
The second starter I put in seemed to mesh good ,no noises or skipping while cranking..but the armature and windings were junk,and it never really cranked over like I wanted it to..it was slow and weak,even with a new battery and cables..the third starter I put in was a new rebuilt from a wreck I found at a boneyard--it was grinding and skipping from the first day I put it in...so I put the nose off the second starter onto that one,and once again it sounded great--that one was still on the engine when I pulled it and sold it...
The main reason most Chevy starters get "eaten" is poor flexplate to starter drive gear clearence..I use a 1/8" allen wrench between the OUTSIDE ot the flexplate teeth,and the armature(the round shaft the starter drive slides on,the motors shaft) as a "gauge" to measure the clearence..I saw this way described in a rebuilt starter catalog at work years ago---it works great,and is much easier than trying to measure it with feeler gauges while you hold the starter drive engaged..you need to be an octopus to do it the way GM describes doing it!..
For whatever reason,the 400 SB seems to be the one with the most starter and flexplate troubles,though ALL Chevy's seem to eat up more than their fair share of starters and flexplates compared to other makes..one thing I now do is keep the nose off a starter that meshed good with no shims,and swap it onto the "new" starter..never had to mess with shims that way...more than one nose I had to file or mill it down to get the drive and flexplate to mesh properly...shimming just the outer bolt does not usually help much,and encourages cracking of the nose in my opinion..
