when you are installing the starter are you shimming to a paper clip diameter space gap between the stater gear and the fly wheel? If not the stater gear will hit the flywheel gear (after several starts) not on the teeth but on face to face contact resulting in the starter gear spinning but no tturning the flywheel.

Also if you are running headers, wrap you starter in heat sheild tape to prevent warm chevy-itis. I would reccomend trying a solenoid designed for a big block which should be a stronger unit.

Warm chevy-itis occurs when the starter an solenoid act as a heat sink as a result of being located too close to exhaust pipe or headers. The result is an engine that cannot be cranked by the too hot starter unless let to cool for 20min to 30min.....

good luck

i would check the voltage that you are getin to the s terminal of your starter maybe when u shake the wires does it start?  

hey man i feel your pain  my 78 blazer had a starter problem like yours that took me quite a while to fix it, my blazer would make a bad grinding nosie sometimes and then it would make the noise you described that it was spinngng but not engaging with flywheel, i too replaced the flywheel and starter twice, with my first starter i tried every sort of shims and would get it to start for like 4-5 times like you mentioned but it would never stay perfect so then i just went and bought another starter and put it in and the truck started up fine with no shims!! the other stater must have been badly tweaked and stuff cuz when i put in this other new one it stated perfectly without any shims and its still working good now  

I remember when shimming starter was a big issue in late 60's and into 70's model years. I am just not sure how it all started in that if it was because of production tolerance slop in bellhousing, starter, flywheel diameter or a combo of all of them but it was a real problem at times and replacement starter in boxes even used to come with shims too.