Tomorrow I will be drilling 11/16" holes in 1/2" A36 with a annular cutter. I will double check the HP but I think it is 1HP, it is a dayton for sure. I have no problems with these holes. I can do the hole in 9 seconds with with plenty of down feed. I have not seen your gantry, but I think you might have to go slower due to down force. With that in mind you could probably get away with 3/4HP? Also consider 90vdc, as you can infinitely vary the speed and they have good torque curves (in your range 1/8-1/4HP).