The "XX" blocks are the later castings upgraded for use in the '78 and '79 "performance" cars. They bear the same "988" casting number, but the date code will be as shown. They also have the heavier bulkheads of the earlier blocks, a weak "link" found in the '76-'77 "557" blocks. Typically, the "extra" bolt holes for the early mounts are not drilled, but the bosses ARE there, unlike the 557.
In answer to the question regarding the bolt bosses, yes, they can be drilled and tapped (3/4 - 1" deep, 7/16-14 thread, use a "bottom" tap to finish the threads). Care must be taken to properly align the holes perpendicular to the crankshaft centerline and "level" (parallel) with the pan rail. The holes must also be properly chamfered. The help of a skilled machinist is advised if you're even a LITTLE "nerveous" about it.
