The mounts are different between 280s vs. 240/260s.
You have to fabricate a new bumber mount to put the smaller 240 bumpers on the 280s. I'm considering fabricating my own fiberglass 240 look-alike fake bumpers to get rid of the ugly 77 bumpers I have now. They'd be lighter too. Someone once mentioned that Arizona Racing Whatever sells a fiberglass version of the 240 bumper but I've never seen it or heard of it but that one time, and I have thier catalogue. Anyway,
There is more to it and in the post that he mentioned it is in it. The site you are refering to is arizonazcar.com I belive. They have all sorts of goodies but he also requires a $1000.00 minimum order.
I did this on my 75, and it was easy. extend two bolt holes on bracket for the 280, use only the inside bolts on the 240 bumpers, fabricate side brackets for the extensions, and deal with the 2 inch gaps between bumpers and sheetmetal. but what the ****, the car is 175lbs lighter, and goes a lot quicker because of it. BTW, its illegal to modify crash components on cars, and insurace will not pay if you have removed original equipment crash crap.