For a heavy duty electric drill, as mentioned above, you might consider a 1 1/2" bit. I like Milwaukee's bit. It's about 6-7" long with about 4-4 1/2" of twist. Mortises more than 5" or so may require attention to chip removal. I believe Irwin still makes the long 1 1/2" bit (16" or so?)and they're decent bits althogh I've never used them for mortises. The narrower shank may lose its trueness doing the demand work of mortises.
For the most part, the critical points of the mortise are at the surface for the purpose of locating the joint. If the sides are cut a little wide (in this case by clean-up with a chisel), it won't affect the joint. The ends of the mortise should be more accurate, especially in a brace mortise.
Using a smaller bit means more hand work, but if that's what you have...... This bit has worked well for me in my boring machine too, with no modification except an extension to the shank if needed for length. Good luck.