Adrian...It is a good idea to make sure there is no crap and old jointing compound at the bottom of the blind holes. Smaller drill or a woodscrew, both hand held, will clean them out. As long as they do not bottom, it is a case of check and shorten to use the maximum thread depth. The bolts should be 1/4 Whitworth Thread, same thread as sump plate studs or rocker cover studs, into the aluminium castings.
Swarfy.