I learned yesterday that the Ecodiesel the Mopar guys are using seems to have a similar setup to the P400 with the bolted girdle and main caps. While it's probably already been discussed and I'm late to the party, I still found it to be interesting. Pretty decent photo below of both for comparison.
That design wasn’t made for the p400.
It is a common design by many different engine manufacturers. The set up we have is a hold over from the 1930s. That isn’t a typo, thirties. As in almost 100 years.