RATIOS

This is only a reference guide... of course... things do happen to cars and rear ends do get changed out. So always measure with rotation. I use chalk make a white mark on the driveshaft. Do the same on the tire if its on, or whatever can be rotated. Place the mark at a point of reference. Like straight up or down. Then turn the drive shaft and count the turns. Do this for one full turn of the wheel/hub/rear half shaft. This will give you a good guess as to what gearing is in the rear end. When you find something close then pull the rear cover and start looking for the marking on the ring gear.

R-200 REAR END SWAP

All you're doing is replacing rear end (and differential) parts with 280Z /ZXversions, that's all. But if you decide to get the R-200 and the halfshafts from a 280ZX, those are the only parts that cross into a 240/260/280Z. The ZX driveshaft and mounting hardware won't.

For a 240Z/260 R-200 transplant, use these parts from a '75-78 280Z:

• the curved rear crossmember which bolts immediately behind the bottom/rear of the differential (early models are straight). The R-200 sits back by about 1 1/2 inches and needs the curve. Bolts in perfectly. This is a necessary part.
• Get the "mustache bar" that bolts onto the rear differential cover from a '75-78. This is a necessary part. Despite your budget, replace the bar's bushings with urethane ones. I tried to be retro, and replaced mine with new soft stock ones, which allowed the extra torque from the 3.90 to thunk the bar-ends against the floor. This made me realize that thunks are only partially caused by the front mount.
• Get the R-200 halfshafts too, halfshafts from all '75-83 R200's are identical. I consider these a necessary part. But be careful not to swap the halfshafts as the flange that slips into the differential on the passenger side is 1/4 inch longer than the driver side. I made this mistake and got 2 short (left) ones from the junkyard. Later ZX's came with CV joint shafts which will fit , but with difficult modifications to the wheel hub which I've never tried. So, try to get the "older style" ones from a ZX. Just "yank" them out of the differential. R180 shafts will just fit as they have to be compressed, but why bother with a "maybe" fit, get the R200's. For some reason ZX shafts sealed the grease fittings with plugs. Remove the plugs and add new fittings, and remember to grease them every 3,000 miles.
• As I said above: to get halfshafts out, DON'T pry them out, that tears up the seals. Simply unbolt the wheel end, and "yank" the halfshaft a couple of times. A little circlip in the diff it but it will pop right out, the yanks don't hurt it.
• Get a good driveshaft from a '76-78 280Z. They are about 20 cm longer than the 240 shaft, which will maintain drive-angle geometry. Strangely, '75 shafts were 10mm shorter and fatter and the rear bolt pattern doesn't fit onto a later R-200, stay away from those. You can use a 240/260 driveshaft, but do it right and try to use the R200 parts. BTW, don't believe the dealer when they say the joints aren't replaceable on your 280 shaft, any driveshaft shop can replace them. It's that Nissan doesn't sell replaceable joints.
• The front differential crossmember from a 280Z that bolts to the frame (holds the front rubber insulator) is thicker and beefier and should be replaced also. Of course you should then replace the actual rubber front differential mount with a '75-78 version.
• The '70' and early '71s differential strap sits forward by about 2 inches compared to later 240 models (different brackets). So if you try a R200 swap on an early cars, the R-200's front mount nuts on the top will hit the strap. I found I had to reverse the bolts, place the nuts on the bottom, and saw off the protruding bolt threads so the mount would flex. I then raised the differential with my floor jack, and placed it very tightly against the strap. Only then was I able to bolt the front crossmember in. Since later 240/260/280s's had different strap brackets, they won't have this problem.

While this may not seem "original", how many more years does your car have on the road? Enjoy it while you can. This is a conversion using factory parts that no-one except the extreme purist would scoff at (or even notice). In fact when other Z people hear what you've done to modify your car, they're usually a bit impressed.