I have a 440 in my 69 383-S fastback and am using 383 mounts on both sides. Works perfect, I am not sure why the factory used different drivers side mounts.
You must be using a 440 that has the two bosses cast into the front of the block like they did on some of the 383's. On the M-Code cars they tapped the two bosses by the oil pump, bent the drivers side bracket a little differently then took and welded a crude piece of angle iron to the bracket. The angle iron then was bolted to the oil pump boss.