For reference, MGBs factory equipped with HS4 carbs had intake and exhaust manifold flanges that were 9/16 inch thick. The later cars with HIF carbs had intake and exhaust manifold flanges that were 7/16 inch thick. Hence, 1/8 difference.

If you might be installing an MGB intake manifold on the MGA, and you happen to run into this problem, you could machine up a set of stepped retaining washers like this:

If you don't like machining, you could glue a 1/8 inch thick flat washer half onto a full washer, and the large, thick manifold washer will hold both pieces of flat washer together.