I'm not sure what setup you're going for but here is some info regarding single and dual disks on the various Kz550's.
As mentioned in the quote, you can't really just put a dual disk wheel with rotors onto a single disk Kz550 and add a caliper. The disks won't align with the pads since the dual disks are smaller diameter. The options are to run two single-disk rotors on the dual wheel or get fork sliders from a dual-disk bike, or somehow make adapter plates and use different calipers. Or possibly get a pair of dual-disk rotors from a bigger bike. The single disk is indeed thicker than one of a dual disk.
It does raise the question why Kawasaki made forks with both caliper mounts if it also made dual-disks with smaller diameters.
Brake disks, and likely forks, depends on the year and location. There are a slew of different brake diameters for the 400/500/550 line.
The Kawasaki Factory service manual, uses the term "effective disk diameter" for it's measurements, which is not the actual O.D. measurement. From what I gather, "effective disk diameter" is measured from the center of the pad area.
From the factory service manual (red base manual 99924-1018-03):
1979 500B1 models:
Europe and Australia got dual front disks. They were 232mm.
Everywhere else got single disk. It was 250mm.
All had rear disk. It was 237mm.
1980 500B2 was the same 500B1.
The forks have bosses for calipers on both sides even if the bike only came with one brake. But the forks will be different from dual disk models because the disk sizes were different.
1980 400J1 model:
UK and West germany got dual disk 232mm.
Japan and elsewhere was single disk 250mm.
Rear drum.
1980 550A1 models:
Europe got dual disk 232mm.
Elsewhere single 250mm.
Rear drum.
1980 550C1 model:
European got dual disk 226mm.
Elsewhere single 236mm.
Rear drum.
1981 400J2
UK and West German dual disk 226mm.
Elsewhere single 236mm.
Rear drum.
1981 500B3:
All got dual disk 226mm.
Rear is 226mm.
1981 550A2 same as 400J2 single
1981 550C2 same as 400J2 single
1981 550B2 same as 500B3
1981 550D1 same as 500B3 (Actual O.D. is 260mm. Rear is 7mm thick, fronts are 5mm thick.)
The forks use different amounts of oil too.