The P400 was discontinued around 2019ish timeframe. It is no longer available new from AMG.
The Optimizer long block is available and is a very good choice. You can see/buy it here
Im leaving the listing up just for informational purposes. THIS ENGINE IS NO LONGER AVAILABLE.
The P400 was the beast in the 6.5 world. Some of the main highlights of them were.
1. Has a full main stud girdle.
2. Forged/billet crankshaft.
3. Stronger metal pour at the foundry.
4. Hardened valve seat inserts in heads with improved cooling passages.