Hmmm...in normal operation, I agree that that the top bolts will be under tension and the bottom bolts under compression, but IMO, when you are trailering, the aux bracket and engine get bounced around a hell of a lot, which will cause tension on the bottom bolts, also. The same applies when the boat is being tossed around at sea.
I would secure the bottom bolts to coupe with both tension and compression.
Have a look at other 520's and see how they have secured the aux.
Bob F.