I have been researching this very thing. Cycle Exchange sells 10mm offset rear sprockets and 10 and 20mm offset fronts. If you use (according to them) a 1977-78 CB750 rear sprocket carrier (just the plate on the left side of the hub, not the whole hub), it offsets the sprocket 10mm further than 69-76 pieces. If you combine the 77-78 carrier and a 10mm offset rear sprocket on a 69-76 bike, you'll need the 20mm front. If you have a 77-78 CB750 (on which the front sprocket is obviously already lined-up with the rear at 10mm further out than 69-76) you use the 10mm front and rear. Their website says this will clear a 180 or 200 rear tire, but this is when using a custom chopper frame and the swingarm isn't an issue. Cyclexchange.net explains this well, with pictures. Also, 20mm is .78 inches, slightly more than 3/4 of an inch
Modern bikes with 5.5 inch rims usually have a 180mm tire from the factory.
Before I came here to Afghanistan for a year, I was looking into this for my bike. Of course, I can't measure it from here, but I remember the space at the swingarm was going to be very, very close to fitting a 180 in there on an 18 inch rim. A 17 inch rim will give slightly more room because the swingarm space widens as it gets closer to the hub (but it will be a very slight difference).
On my 78 swingarm, there is a lip at the seam that protrudes inward toward the front edge of the tire. I personally (many disagree) think it would be fine to trim this lip back where the tire hits it if the tire would fit without removing any material from the actual swingarm wall. 69-76 swingarms are, I think, narrower, but I'm not sure. A lot of people have swapped swingarms from CBR600s, etc in for the extra room for a wide tire and had eyes for the dual shocks welded on. A stronger, more modern swingarm may let you better be able to use some of all that traction available with the modern tire, too.
Remember, you'll have to bend the anchor arm for the rear brake out of the way of the wide tire, too. Some have said that a wide tire will contact the tops of the shocks, too (even Carpy told me this), but I can't for the life of me see how.
I know you said you're thinking of using a modern wheel and therefore don't need a lot of this info, but there it is, in case you decide to lace a wide rim on your stock hub and use that instead.
-Sherm
