If you were in my shoes, what would you do?

[78F350]

I'm guessing that your research has lead you to the decision that it's LN or nothing? It does clearly seem to be the best retrofit. If I didn't have anything else more fun to do with $3-4k, I'd probably go ahead and have it done. I usually have other things I'd rather spend it on though, and after I spent the money, I'd still probably worry about having one of the other failure modes on my (now more expensive) engine.

Are you sure that the engine has to come out on a tip? Once the flywheel is off, I'm not seeing much difference between the jobs. Of course, with the engine out, it's an opportunity to also get the chain ramps and some other wear items.
Personally, I'd change the IMSB myself and choose one of the lower cost solutions. "Better than the original bearing" is good enough to get me down the road another 50,000-70,000 miles 'till I fell like doing it again. Also if something fails, I would not worry about keeping a matched original engine unless I had one of the 550s.