Thanks for your feedback!

Sorry, I failed to mention that the BB shell is a 68mm shell (I measured it) and this is a MTB setup. The currently installed BB is a BB51 which is what the M612 calls for. The T551 is spec'ed for a BB52, which, by my understanding, is the same as the BB51, just upgraded and replaces the BB51 in the Shimano lineup.

I've thought about just removing the NDS-side BB spacer (2.5mm) since I have plenty of room between the crank arm and the chain stay on the left side. I wasn't sure if this was a good idea or not (changes the symmetry, but the chainline doesn't get changed). By the way, I took the T551 crankset off the bike and threaded the fixing bolt onto the spline to eliminate any doubts about whether it fits or the threads were messed up. No problem with that.

I knew this was a Trekking crankset (hence the "T" in T551), but when I looked at all the drawings and the current BB setup, I couldn't find any reason that it shouldn't work. I did have to change out the front derailleur to accommodate the change in the chainring tooth counts, but obviously that has no bearing on the installation of the crankset (with respect to the fixing bolt). I was hoping I was just forgetting to do something that was obvious. If you have any other ideas, I appreciate your help.