i have the same question, a guy told me that it could be done , but that it would be a lot of work. looking through parts catalouges, i wonder if the early 260 bumbers would work, as they have shocks, and i know the late 280 bumbers bolt right in, as my car has that. the other thing is the 280 has a large space for the bumber and you would have to cover that up, have you seen the motorsport areo II kit, it is a nice alternative. also remember that in the event of an accident your bumber will do something, and the earlier bumbers will not.
I kinda agree with Mike.The 240Z style bumper will fit right on but you need to use the brakets that also come from the 240Z. You can get the bumper with brackets and everything from All Z East for $200 (and it will be in good condition, they said) I want to do the same thing with my car but dont have the $ so I 'm still checking the junkyard on a regular basic. Let me know how it comes out ok.
The 260 bumper can be done alot easier. As for the space between the bumper and the car just compress the bumper shock and weld it or drill a hole in the shock ( after its bleed) and put a bolt in it to hold it where you want it. A 240 works also but it is a little more work. I heard you can use the stock 240 mounting brackets by turning them over or around. You would take the shock struts out though. hope this helps
In the next couple of weeks I will be doing the early 260 to 280 conversion and photographing it for my web site. It is pretty much exactly as described by Sizzler but some body work is required. You have to remove the sight shields in the front and back and in the back you have to fill the holes left and repaint. On the sides in the rear you have to fill the body section that is indented and figure a mount for the bumper sides. Can be done. Not automatic. Loses about 100# of fat ugly bumper though.
I put the early 260 bumper on my 78 280 and it was fairly simple. I drilled a hole in each shock to release the pressure then mounted the 260 bumper. I then pushed the bumper in till I got a nice close fit to the rest of the body then welded the shocks in place. I used long bolts on the very end of the bumper (at the side of the car) to keep the trailing edge of the bumper from bouncing (you'll see what I mean) As far as the back goes. I just removed everything and smoothed over the whole back end. It now looks like it came that way.
BTW I painted the 260 front bumper to match the body colour.