99 block will be an 8 bolt bell-housing. It's the only 8 bolt dual cam N/A out there.... Well till you get into the H6's and newer FB/FA engines, etc.
It's really a 251 case/crank/rods with 25D pistons. A little trick on Subaru's part.
so does the EJ251 have a shorter stroke then the ej25d, and thats why the lower compression in the frankenstien setup with the 99 block vs the earlier ej25d blocks? (piston doesnt come out of the block as far on extension)