C182 pilot reported a NMAC with opposite direction traffic while in cruise flight due to a substantially different altimeter settings.

Narrative

I was flying south from ZZZ to ZZZ1. I requested flight following shortly after departure from ZZZ; and when I checked in with flight following the controller gave me a local barometer setting of approximately 30.01 After another hand off an Approach Controller cleared me through the class bravo west VFR transition. It was very busy on frequency. Aprox 20 miles north of ZZZ2 I was initially instructed me to descend to 6000 ft; but this was then amended to 6500 ft. due to traffic. I leveled off at 6500 ft.; and was asked to look for traffic. The traffic reported me in sight; and I had visual contact with traffic. Due to frequency congestion I was unable to report this to ATC. Initially it appeared that the traffic was not in direct conflict and would pass below me with no evasive action required. As the traffic got closer they then reported getting an RA. By my estimate they passed below me with 400 feet vertical separation and 100 feet horizontal separation. Shortly after I was handed off to tower; and had time to get the ATIS at ZZZ1. The local barometer setting was estimated to be 29.89. I feel this likely contributed to the 400 feet of separation instead of 500 ft. due to the incorrect setting of 30.01 being used at the time of the conflict. I was not given a local altimeter setting when checking on with the new controller; and did not have time to get the ATIS at ZZZ2. Exact barometer settings are based on my recollection and may be slightly inaccurate. The remainder of the flight was uneventful.