Finally, some pictures. Had to find these at -18C this morning with a stiff wind..
Here is a regular regulator from a 74-76 260/280.
Note the deep thickness of the door mount flange area, ~3/4". The 240 regulators have a much shallower flange area, more like 1/4"
Other wise they are the appear very much the same. Other than the improved plastic backing disks on the ends of the scissor arms. 240's have little crappier ones that are commonly broken...
You can use a 240 regulator on a 280 door if you use a couple of washers at the four mount positions to space the regulator back to the same position that this larger flange has. Using a 280 reg in a 240 door is more problematic.
There are also diiferences in the two little u-channel tracks on the window itself that the reg rollers move in between 240/280. One of them have shorter channels. If you mix up windows and regulators, you might find your window falling off the tracks!
The big SS frame that the glass seats in when rolled up are also different 240-280. The mount points are different. Just try to put one in the other's door and you'll drive your self crazy trying to get all the 6x1 bolts in. They seem so close to lining up, but they really don't.... Clearly they were trying to make the windows work a bit better (or clear the bigger intrusion bars or something) so they had to tweak all the components just enough to confuse the hell out of all of us guys trying to get stuff to work 40 years later.
The 77-78 regulators, glass, guide channels and window frame are all totally different. Does not fit or work in anyother door. Too bad, they are far far superior to the early stuff.
Some day when I have time I do a proper picture tutorial of all the differences.