It's no wonder the Slant 6 is so great!

To answer the OP question with rumor only....

There is a post somewhere that the block was over-designed and used excessive thickness and webbing due to the possibility it was going to be an aluminum block. Don't know if its fact, but the block itself is quite a beast.

The unequal runner length of the manifold is helpful for wide range fuel of operating conditions (short runners and long runners) so there is some advantage for even cylinder balance (even though fuel equality is not the same). Some life advantage there.

Combustion chamber flow is basically a u-turn, very bad for higher rpm power but great for regular driving use which gives the durability.

Lastly, no one expects it to be sporty and therefore is driven as such.

Just my 2 cents or less.

RGAZ