so long story short, if I show up at mission for the first time will I fail tech with this belt?
No.
I asked Darren K about this (he went through this years ago). It used to be that your stock 40 year old belts were good until you went faster than 11.999. Then you had to have a roll bar and 5 pt belts, and the belts had to be certified every 2 years. If you stuck the roll bar and 5 pt belts into a slower car, it didn't matter, every 2 years.
Finally they figured out this silly rule disuaded people from buying good belts, so they relaxed it. About 3 years ago they also moved the cutoff from 11.999 to 11.499 requiring the roll bar and belts. And, if you are slower than 11.499, your belts are good indefinitely as long as they are in good shape. Faster than 11.499, recertification is required every 2 years.
I recall the belts Rick found for his 54 have 1973 dates on them! Since the car is only 12s, they are good for many years to come.