OK thanks gents, good points.

In this case I'm not referring to any specific block, just a general case, so no pics.

I will add this, to the point of changing the mount location on the block adding stress,
that is exactly what is happening when using a front motor plate.

As far as welding a steel plate to cast iron, I thought you could safely do that, maybe not.

Personally speaking, the resale aspect doesn't concern me, but that is a good point.

What if you wanted to put a Hemi in an old truck, would you use stock Hemi mounts in the stock location ?

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I think the outside wall of pretty much any block is not attached internally to anything, just top & bottom, front & back.