If you want to stick with original hubs, then it depends what size rims you want. I presume your sticking with the 19x1.85 on the front? Thats as wide as your'll need on the front anyway. The bigger front tyre adversely effects handling. The standar rear rim is 18x1.85 which is fine up to 400. But really, a 18x2.15 would be a better choice, from any sohc 750 (apart from k7, i think) The k7 used a 17x2.15 which would allow you to fit a 130 max.
Even after getting a wider rim, I wouldnt personally be too concerned with squeezing the biggest tyre I could on the back, a 400 or 410 works very well if its not pinched by a narrow rim.
If you get a chance compare the way tyres 'appear' on a wheel, say a 400 fitted to a 1.85 and the same fitted to a 2.15. There's a big difference in profile.
Hagons or central wheel components will fit any rim you want to your hubs, they'l just drill a blank rim to suit. And they can powder coat or anodize too. But its not cheap and turn around at the moment is about 6 weeks.
This guy is very helpfull as well.
http://www.cwylde.co.uk/section.php?xSec=3cheers