[Formulajones]

Not exactly sure on a 69 Bird but I have a 69 Z/28 and would assume the tail pipe hangers are very similar.

On the camaro's, a factory dual exhaust car had an extra plate welded to the side of the frame (unibody structure) on the driver side that matched the passenger side. The factory hangers bolt to that. I'll post a picture of it.

Anyway I'm not aware that they make these hangers to fit anything larger than stock tailpipes on these, at least in my application anyway, which is okay, because I had the same drone issue you have with an aftermarket Pypes 2 1/2" system and transverse muffler. So what I did was I grafted the stock 2" Z/28 tailpipes into it. This solved 2 issues.
1) They now fit in the stock hangers
2) It toned the noise down to the point the drone is now gone.

This probably isn't an option for you but will give you an idea how the factory hung the tail pipes on a 1st gen 69. Technically the clamp on these hangers for the 2" tailpipes is just spot welded onto the hanger. You could drill the spot welds and make your own clamp to fit the size tailpipe you want to run. Just a thought.