Need to find a starter after swapping an Allison transmission from a manual to automatic?

The starter bolts to the block and it's the same part for a manual or automatic GM
10465167. What is the problem with the ones you have tried? Grinding, won't engage?
Replacement shouldn't be difficult as you just remove the harness, then the two bolts and lower the starter.

Okay, so you bolt it on and hit the key and it just spins free? Or does the starter pinion hit the ring gear and grind instead of sliding into place? What happened with the original starter? You say this was a swap, was the Allison from another 8.1? Just trying to determine where the issue is. GM used a single starter on the 8.1 but they used a different flex plate on other uses of that transmission. The one you need would be a 14.5" 168 tooth part.