The factory bolts for mounting the 'carb plate' to the case are #10- 24tpi. I would suggest that you find some allen heads bolts that are longer and trim them for max length including the thickness of the engine mount and do not forget the LOCKTITE. Ask me how I know on both items. BTDT

AFTER having bolts vibrate and break the thread sleeve out of a blindnut on the test stand, I decided to put a nylock on behind each of the 4 #10 bolts that hold the motor mount to the firewall.
(I have now had all three blindnut 'spikes' shear off. I'm liking the nylock locknut idea even better! )

1/4 may be a bit large and if it bothers you enough, run a countersink on inside part of the holes fill with epoxy and a filler (talc if nothing else is available) and redrill smalller hole thru the filled area. That will make a H#$$ for stout holes in the firewall.