A 1977k Regal two door will fit on a 1976 Regal two door.
Not without modification
Any other window from a 1975-79 Buick Skylark, Oldsmobile Omega or Pontiac Ventura/Phoenix 2 door would work
NO
DJ's (Dispatcher postal jeep by AM-General)had hardtops and sliding doors those years didn't they? There were DJ's that were basically 2wd CJ-5's early on though. CJ-5 was redesigned in 1976. A 76 top will fit newer models, but not pre 76 if both are CJ-5 style jeeps. Early ones had a channel on the windshield frames for door flap, not on the 76 and newer which had bigger, more raked windshields.
There are three numbers that fit those requirements.
eBay
yes it will
Yes.
I think 1 x 76 = 76 are the only two numbers that form 76 when multiplied together.
yes
no such thing as a 76 hornet frame
Yes they will interchange; everything behind the front header panel will interchange on those two cars.