Thermostat housing on a LD 340 intake

You must be talking about the head sizes of the bolts- bolts are sized by the shaft diameter. Unless you meant a 5/16 bolt?
Anyway, LAs had two different size thermostat housings that are not interchangeable, a larger diameter one that was used up until the mid '70s, and a smaller diameter one that was used from the late '70s onward. but to my recollection, they still used the same size bolt...
Are you sure the LD340 didn't have stripped bolt holes that were drilled out oversize?