jack stands

How on earth did you figure that out?

The OP gave no information on his jack stands. None. So how can you even begin to guess what size bolt will work? Are these 3 ton stands? 12? 20? 1?

I mean absolute minimum you’d want the shear strength on the pin to match the jack stand rating. Realistically the stand is probably gonna put the pin in double shear so that would be an additional safety margin. But we really don’t even know that without knowing what kind of stand this is. The one I posted a picture of is just an example. And if he’s got that kind, he should recycle it and buy something less likely to kill him.
The car is a 65 Barracuda that weights around 3000lb. The most any stand will need to carry is 700 to 900lbs. You might be over thinking the situation