I have run the Shinko 230 -150/90 15 on a 3.5'' aluminum rim and the bolts for the side rails had to be switched out with button heads. The 777 should be the same size. My first one was 5 7/8'' wide, my third was 6 1/8'' wide. Still cleared the belt and guard. If you are using the stock rim (2.15'' wide) you will be narrower and have a crown in the middle and a slightly un-natural profile. Try running a little low on pressure 28-30 lbs psi. It is a HD tire as I recall so it is stiff by it self. I had not checked the pressure in mine for a while and noticed it feeling mushy, it was down to 14 psi. I normaly ran 28. Slowly going down thru the 20's I never noticed a thing.
Being crowned will cut down on it's mileage, but with a bobber you won't be running 20k a year. Nothing aginst bobbers, but most shown on this site are several years old and still have the stock rear tire. (good for 7k at the most)
You won't be able to put a 25 tooth pulley on to raise the gearing or the tire will be REAL close to the front of the swingarm. But the bigger tire should correct the speedo error.(check with a gps app on your phone)

When I fitted a 140/90/15  shinko 230 to the  rear I worked out the diameter was increased by 4.2%
Checking the speedo against a gps after fitting the new tire they read the same.
Made a difference for the better to the highway riding as well