widest tire for a Z1

I think you may have to go with a custom swingarm.

Widest you can go without chain to frame clearance issues is 180 rear with a 5/8 offset front.

Use a zxr750/zx9r wheel (5" rim width for 170 - 5.5" for 180 depending on model year) for an easy life as the Zed's 630 sprocket bolts straight on these and take 8mm off the sprocket carrier face to line it all up

With a 630 chain there's only a couple of mill clearance between the chain sideplates and frame and like wise the tyre but i've run this for 10 years with no prob's.
Better still convert to 530 pitch while you're at it and get more clearance.

You'll have to compensate for the wheel dia change and 1.94 to 1 is closest equivalent to the original zeds 2.2 to 1 ( with 18 in rim ). I'm running 19/40 gearing with 530 and 17/33 with 630.

You will have to use a wider arm and most popular graft for twin shock is ZRX 1100/1200 with pivot and shock spacers or XJR1300 with some shock spacers and pivot machining or 600/750 SRAD with pivot spacers and monoshock etc .

I am using a 140/90-16 on the rear of my 1982 KZ650 CSR. The rim width is 3". Originally, the bike had 130/90-16. The 140 fits, but just barely - there is only about 5 mm. clearance on each side.

I would have preferred a wider wheel for more stability, but it would have spread the 140 tire so that I doubt it would have cleared. 130 is a safer bet for clearance.