I went ahead & did the search for 'ya, damnOptions for Centered Rear Ends
in FJ40 Landcruisers
Stock Offset FJ40 cruiser rear end width WTW (Wheel To Wheel)= 1400mm=55.1" (inches)
I have not measured the WTW myself but this should be pretty close
6 lug semi-floating 30 spline Dana 60 (55" or 58" WTW)
You could do the 6 lug semi floating dana 60 using toyota pickup axles, brakes, and a portion of the toy tubes with a dana 60 center section and tubes. The width depends on the year of the rearend; 55 inches wide for pre-IFS rearends and 58 inches wide for IFS truck rearends.
V-6 or turbo toy pickup rearend (55" or 58" WTW)
You could just use a v-6 or turbo toy pickup rearend. The toy pinions are the same size as the cruiser's at the splines. Also, the axles are the same at the splines except they do not use c-clips. I heard that both (maybe only one of them…can't remember exactly) the V-6 and the turbo toy pickup rearends have stronger third members than the standard 4 cyl rearends. They have a stronger 4-pin design carrier.
9 inch rear end: late 70's Lincoln Versailles.
You could use a 9 inch rear end out of a late 70s Lincoln Versailles. It is approximately the right width. They are hard to find, use 28 spline instead of 31 spline axles, and are 5 lug (you could have the pattern changed or get new 31 spline axles with the correct bolt pattern).
9 inch rear end: '66-'77 Bronco (58" WTW)
A 9 inch rear out of a 66-77 bronco would fit, although it would be about 3 inches wider than the stock rearend. These are 28 spline so new 6 lug 31 spline axles would be in order.
Dana 44 or solid axle AMC 20: Wagoneer (58" WTW)
Another rearend you could use would be a Dana 44 out of a Wagoneer or a solid axle (not the splined hub type) AMC 20 out of a Wagoneer. However, it is difficult to find the centered AMC 20 that has solid axles.
Dana 44: Scout or Scout II
A rearend out of an International Scout or Scout II would work. They may be a bit wide though(4-6 inches?).
35 spline Dana 60 (semifloating).
Also, you could make a 35 spline dana 60 (semifloating). The housing ends are about $65 a pair from Art Morrison or Dutchman or Moser. The bearings would probably be about the same, if not less. The axles cost about $400 but are probably the last and only ones you'll ever have to buy for that rearend (Dutchman has 40 spline Dana 60 axles with bearings, retainers, and studs for $470).
Stock centered cruiser housing (1958-1962) (53" WTW)
Also, I can't forget the stock centered cruiser housing. Drawbacks: hard to find, uses 10 spline axles, and is 2 inches narrower than offset cruiser rearends.
Custom centered cruiser housing (55" or 58" WTW)
Custom centered cruiser housing ($350-400) using toy pickup axles and brakes. A guy in Oregon makes these housings. I think the guy is John Groves of Cruiser Parts in Grants Pass Oregon. Phone is (541) 479-5500.
One last thing about cruiser pinions vs pickup pinions...the pinion splines and diameter of the splined part of the pinion are the same (fine spline ones) because I'm using a pickup flange on my cruiser third member so I can use the toyota pickup driveline which has a longer slip yoke. The rest of the pinion is different