I beleive there have been a few 500 caddies swapped into first gen Zs, dont remember any names off hand, though. Answer to the first question is no, the 260Z and 280Z have essentially the same engine bay. Unless by 280 you meant 280ZX, someone else would have to chime in on that one. Stock rearend in a 260Z should be an R180, which would likely go boom behind a stock 500 caddy. An R200 is an easy upgrade, but I seriously doubt it would stand up to a built 500. You could do the R230 swap, or convert it to a solid rear axle like a 9".