There isn't much room as just beyond the plugs are the bottoms of the jets when they are screwed into the main casting.
A few years ago I started using 10-32 set screws, and only tapping in a few turns so they would seat tight and flush with the casting surface. They are pretty short, apprx 1/4" long.
We sell the screws to seal up the bottom plugs separately, to do the front plugs and the larger rear ones as well. $4 for the set, beats having to buy 50 or 100 of them from a fastener store!......Cliff