What do they mean

Because the number starts with a 3, it means that the driver claimed Division 3 as his home division, most likely in NHRA. It may include Illinois; Indiana; Kentucky; Michigan; Ohio; Wisconsin; and Western Ontario, Canada. I don't know if NHRA had any different states in Div. 3 decades ago. The only driver I could find who had 326 in Super Stock was Craig Watts from Stoney Creek, Ontario, Canada around 2019. His car at that time was a GT/LA Mustang. But it's possible that the driver of the Valiant retired and that number was assigned to someone else afterwards. Other classes like Stock, Super Gas, etc. have their own separate set of numbers, including 326.

Does your car have any "local" decals or sponsors that may have a phone or address? That could narrow down the area that it's from and possibly provide contact info to any local companies that may have been associated with the car. Most of the decals on the car are "contingency decals". These are companies that have a marketing/advertising deal with NHRA that if a car wins or gets runner up at one of their races while using their parts and displaying the proper decals, they get paid extra money from those companies. (Contacting the contingency sponsors probably won't yield any info.)

What's that big round decal above the rear tires?

You'll have to sign up to post, but there is a racing site that will probably have people who may recognize the car from way back - whenever it was and be able to provide some history on it.

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