It is not uncommon for 757 flights from MIA to be swapped out with the 767-300. I have worked those flights several times. Even an A300 once. I know DL used to fly several 767's and even L1011's back in the day out of LGA. A quick search on airliners.net or jetphotos.net you will find widebodies by almost all the legacy airlines back in the 70's and 80's.
Delta was flying 767s (-200, -300, and -400s) on a regular basis to LaGuardia until around 2003. They had 767 flights from Atlanta, Cincinnati, Fort Lauderdale, and Orlando.