I think it was at the Proving Grounds (again) in '85 (I could go and look on the shirt !!)
You are right.
1981-Chelsea, Michigan, Chrysler Proving Grounds/Milan Dragway: 500 cars, 1,500 participants, and seven vendors-five of which are still with us today. The event was closed to the public because of its location. It was the first event of its kind, with Mopar car clubs across the country invited to attend. Since it was a gathering of Mopar car clubs, it came to be known as the Chrysler Car Club Convention. Direct Connection served as the sponsor. Car judging and drag racing were the highlights of the event, along with individual car clubs and certain car lines posing for pictures on the high-bank oval.
1982-St. Louis, Missouri, Mid-America Race Track: 500 cars, 1,000 attendees. The Scat Pack Club, National Hemi Owners, and SIAC clubs combine efforts to produce the event. The car show and small swap meet were held in the hotel parking lot, with the racing activities held at Mid-America Raceway.
1983 - Chelsea, Michigan, Chrysler Proving Grounds/Milan Dragway: 1,200 cars, 1,500 participants. For the first time, the drag racing portion of the event is open to the public, and the show debuts the name Mopar Nationals. Carrol Shelby is the guest speaker, along with the original members of the RamChargers and designers of the NASCAR Wing cars. Debut of the Shelby Charger 2.2., track tours, autocross, and drag racing are the event activities, plus an organized photo session on the high-bank oval featuring same body/models shot at one time: AAR/T/As, Daytona- Superbirds, Hemi power, B-Bodies, and A-Bodies. Twenty-seven car clubs participate. The Dodge Daytona and Chrysler Laser debut.
1984-Indianapolis, Indiana, Indy 500 Track: 1,500 cars, 4,000 participants. Produced by the newly formed Chrysler Car Club Council. Midway and swap meet grow in size and are located across from the track, while parade laps and show and shine are held inside the infield of the Indy 500 Track. Miss Direct Connection and Chrysler concept cars are in attendance. They had a showing of the Shirley Muldowney movie, Heart like a Wheel, and a Herb McCandless seminar. This was the only year drag racing was not held in conjunction with the event. We learned our lesson, drag racing had to be a part of the Nats.
1985-Chelsea, Michigan, Chrysler Proving Grounds/Milan Dragway: 1,850 cars, 9,000 participants. The Nats debuts the Grand Prize give-away car-a '69 Plymouth Road Runner-night cruising, exclusive Mopar-only drive-in with the movie Christine showing.
Information extracted from this article chronicling 25 years of the Mopar National:
https://www.hotrod.com/articles/mopp-0508-2005-mopar-nationals/