2&1/8" tti in 73 duster

I have a SBM on the 72down K. I had to shim the bisquit mounts for clearance..
Your BBM and 73up mounts are a little more difficult to adjust, but should be well worth the effort, especially considering the price of the alternatives. And the TTIs are loooong lasting, and mine hold their flange torque very, very well.
Good luck.