Thanks! Yeah, I'm building an 81 XS street tracker. I know an "official" tracker has 19s front and back, but it seems about impossible to find 19" rear rims unless you want to spend a bunch of money with Buchanan to have a custom drilled rim and spoke set, so my options are 4.0 x 18 R and 3.5 x 19 F or 4.0 x 18 on both. I think I'd rather have the fatter tire, but your red bike looks damn good with the 19" K70. I like the K70s; I have a Harley Sprint street tracker that I finished last year (my "Red Bike" avatar photo). I have 4.0 X 18 K70s front and rear on it.
BTW, my XS tracker is going to have a blue / black speed block scheme similar to yours.