BMEP was used to verify power output, and on the DC-6, if you had the same MAP and RPM on all 4, and one BMEP was noticeably lower than the other 3, that was an indication something wasn't right
The operative word is "noticeably." In the Convair (assuming my memory is correct from over 50 years ago since I flew the Convair 440) there was usually around 15 BMEP difference between the two engines on take off. That was because one of the engines was used for operating the air-conditioning /pressurisation systems and that stole power from that engine, for want of a better term. I forget which engine. However manifold pressure and RPM indications displayed equal for both engines. It was only the BMEP for one engine that was slightly down on the other engine