Terry, 5/32 is only a bit over 1/8 inch which, when I visualize that, sounds very thin for quite large areas cast in sand. Yet even in the Victorian era, they were able to produce some amazing cast objects.
Are your walls thicker than this? Is that where some of the extra weight is?
I was watching 2 Burtz blocks being cleaned and deburred in my workshop by Dave Mossman and a friend yesterday, but I did not look at thicknesses.
I did see that your cast iron is finer grained and harder than the original block, which was sitting on the bench right next to your two. Some of the Ford original castings, like manifolds, are really soft and can be sanded flat very quickly by hand on a granite surface table.
My 2 Burtz engines are still in transit somewhere.Not in a hurry for them though.
SAJ in NZ