I’d have to agree with The Old Magnet has posted here.
With a two standoff approach floating in space there are major problems in the kinematics or lack there of beyond taking care of the inherent vibrations that Old Magnet has mentioned. There is nothing to control the yaw axis movement. You are relaying on the accuracy of the spot face on the two mounting standoffs to control that movement. The lower die cast hole in the Alternator being longer might give you some yaw axis control, but that hole is seldom coplanar with the axis of rotation of the rotor.
Anyone who has worked with GM automotive starters knows why they sell eccentric studs and tapered shims to go with those starters. The alternators are not any better in terms of mechanical repeatability as there is a huge variation in the mounting arrangement that must be adjusted for axially. These GM housings are poorly die cast but in the original application that is not as much of an issue as a belt driven arrangement can mitigate a lot of axial and angular misalignment that goes beyond what a Love Joy coupling can accommodate.