If the GEN2 (passenger side) light has not come on, then the rear battery has never been charging. The stock starter needs the voltage from the front and rear batteries to start.
If the GEN2 is not working then you CAN run the truck for a long time and get many starts until the rear battery doesn't have enough juice to crank the starter.
Yes, NAPA does sell the correct rebuilt alternator, but there have been some people buy them and find out that they haven't been rebuilt correctly. If you buy one make sure that the ground stud is indeed isolated from the altenator case. I have paid $85 to have one rebuilt and usually pay $45 for parts and rebuild them myself.
There has been many posts on the correct starter bracket numbers. I have posted the numbers and pictures for both brackets. Old Direct Drive style and the newer Gear Reduction style.
Before you start any troubleshooting, charge your batteries and make sure they pass a load test. If they don't then you will be chasing many issues.