I'm not a structural engineer, nor do I play one on TV. This has issues - IMHO.
While I like the bushing idea for vibration dampening, I think this sends at the very least some visually alerts off in my brain.
1) I don't think drilling through the down tube is ever a smart idea. Maybe if it was a hole alone it wouldn't be an issue but you've got a buzz saw on a single bolt throught the hole.
2) Is that another hole (smaller) perpendicular to the mount hole?
3) You have the dynamic mass almost at the center of the chord.
So I put this all together and it seems you have created a fatique point and are purposely reefing on it with engine movement
I hope someone tells me I'm full of poo and you are safe.