Hi.In my yout,I took a '70 750 down a slightly curving staircase of average width to the basement as follows;removed mirrors,tank, seat,rear fender and tailight assembly,and exhaust pipes,left installed flat bars on as not as wide as stock to begin with.Bar width may be an issue for U depending.Myself and a friend then walked it down the stairs,me in front working brake and pal in back slowing things down holding frame at fender mounts.
A subsequent winter rebuild on same machine was much easier!Took it piece by cleaned piece to my apt on 9th floor and back down elevator assembled in spring missing only rear fender assembly. Have Fun and Rock On