I disagree with the statement (I have heard this before) that the slant six was designed as an aluminum engine, and therefore the cast iron version is much stronger.
Okeh, so you disagree with what the actual, real chief engineer of the slant-6 has to say (in great detail and with strong documentary evidence) on the subject. That's a little strange of you, but whatever. Have you read Weertman's books?
I have looked at both blocks. If you compare the aluminum block to the cast Iron block, there are many differences.
Ah, I see your error. What is said (and supported) is that the slant-6 was originally designed to be an aluminum block, not that the slant-6 was originally designed to be the specific aluminum block as it was produced from '61-'63. What is meant by "designed to be an aluminum block" is that the basic geometry of the enginethe deep skirt, etc.was conceived and specified so as to be compatible with an aluminum block material. That, in turn, means the iron block made with that geometry is exeptionally rigid and strong. That's all.