There's a GMC 270 engine listed on ebay at the moment that seems to have a serial beginning ACR or AOR with a rebuild tag headed RCEME. Is this of any use to you?
http://www.ebay.com/itm/270-GMC-LONG...E:B:SS:CA:1120
All standard disclaimers about no knowledge of the seller apply but it is possible they are an arm of Sam Winer Motors (surplus dealers).