The 40SC cam fits right in the 48's and 52's. I have two 52's with the 40 SC cam. They seem to run just fine.

The only difference between a 48 and a 52 head is that on the 52 the glow plug is dropped down into the combustion chamber about 1/8". It's worth about 300 rpm try one head and then the other. You can see this from the outside when the heads are side by side.