There are ways to shorten the crankset. I searched for a service but was unable to find the one I remember from earlier in the 2000s. There are also adapters, but they are rather pricey.

However, I did come across this video, which shows a fellow using the most basic of tools in order to accomplish the job.

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I don't know if it would work on your Sugina crank arms or not, but it is worth exploring in order to gain more ground clearance--- and avoid pedal ground strikes--- which are rather scary!

Great story, pictures, and build! Since you mention not doing much MTB riding, consider replacing the knobbies with smooth tread tires. If larger tires can fit you could gain a little bit of BB clearance.