I put the '72 and earlier 240 bumpers on my last z, a '77. Its been at least 3 years since I did it so its a bit hazy, and I don't own it anymore. I guess it was time consuming more than anything. You'll need both the front and back 240 brackets as a starting point. The front was fairly easy. I just flatened out the bracket so it was L shaped instead of the two angles. One of the stock threaded holes which the bumper shocks originally bolted to was used, and then I drilled another hole for the second.

The back was a bit more complicated. I ended up using the tie down hooks as a starting point. They are positioned fairly close to where the bumpers bolted up on the 240. A bit of an extension was welded on and I ground off the hook part. But looking back the hook probably cold have stayed on. The 240 brackets were then welded to that. The holes in the 240 bracket do not line up with the 280's, so I had to be a bit creative cutting the stock brackets.

There's probably someone out there who did it better and more efficiently, but that's how I converted mine over. Never regretted it either!