I put one of these on out '66 122S and had much the same struggle you did. The center hanger ends up canted way over to one side, and while I was able to tweak everything so nothing rattled against the floor, it would still bang into the floor going over any sort of substantial bump. I ended up wedging a block of hard rubber in there someplace so it can't do that anymore (don't remember exactly where at this point, but it's against something that doesn't get as hot as pipes and mufflers). That's stayed good for years.

I didn't like seeing the rear muffler all that much and the boy racer tailpipe doesn't suit the car either, so I replaced the rear muffler with a Thrush turbo that tucks up much tighter, and put a chrome tailpipe with a turn down on that. It does use a strap hanger hooked to a bumper bracket bolt, but that's held up fine too.

FWIW, I've put Simons on various customer 1800s, and those are a much better fit. The only problem is that the rear hanger is a rubber bungee that doesn't hold up for long -- a strap works fine as a replacement.