You don't need a shear plate for an internal linkage Dominator, you do need a safety plate, if you use an open spacer to make sure if anything happens it don't get in the engine. A 4 hole spacer does the same thing. 20ish bucks from summit or jegs

As for which spacer you need, there is no magic formula. If it is a street strip deal Id likely just use a 4 hole, wont need the safety plate that way. As for which will work best a lot of factors in that equation, as stated only way to know is try both. One will give you a better signal to the booster the other ads to plenum volume.