Finishing up a Dick Sarpolus Mustang and had a question about the mounting of the engine. While the mounting holes will fit a 4-40 screw would a 2-56 be a better choice to go into the 1/4 square motor mount?

IMO a 2-56 is perfectly adequate for 1/2A engines.  If the holes in the engine are too large a washer may be needed to keep the screw from pulling through.  I bliessed with a lathe, so in extreme cases I make a stepped washer.  3/16 OD, with 1/32 thick flange, boss diameter to match the hole in the engine with length just less than the mount lug thickness, then a through #44 (.081)  for the mounting screw.

Philip, glad your getting along with the Mustang,the metric engine to tank mount screws were very similar to 3-48 in size. I generally use 4-40,  2-56 might be the better choice for the 1/4 inch mounting wood, i have used 2-56 quite often i just like to check the tightness a little more often with that size just for peace of mind! Hope it turns out great for you!

I've used both 3-48 and 2-56, prefer the smaller as its blind nuts fit a 1/4" wide mounting beam better. Smaller hole surely makes for a stronger beam too. In my experience 2-56 is plenty beefy enough for 1/2 As.