Troubleshooting the problem before you started replacing parts would have saved you some time & money.
I assume that the starter motor turns over, but does not turn the engine over. Is that correct?
The ring gear is slipping, the ring gear is missing teeth or you have a bad bendix that wasn't replaced when the starter was rebuilt.
Front or side distributor?
If it's a side distributor, look through the timing port for a loose ring gear. If it's a front distributor, pull the starter to check.