Just the lower two studs, the upper two are bolts and you can buy whatever length & grade that you like. For my lower studs, I bought a set of bolts that were far too long, and used the lathe to trim & thread em. Not sure if we heat-treated them after or not. Possibly, but not scientifically. I wouldn't be overly worried about it. Use the rolled threads in your engine case, and the cut threads on the chassis-side.
-Dave