1/4" stainless carriage bolts, 304 passivated stainless, minimum yield strength of 45,000 PSI.

Each bolt has a minimum yield strength of 2,209 pounds. Even at a fraction of that strength, I've got six bolts holding down my load (three hold downs per machine, two bolts per hold down)... I think I'll be fine.

I'll use either a pair of large washers (fender washers) or a square metal plate on the bottom side to spread the load out across the plywood floor, which I suspect will be the limiting factor in the amount of load each hold down will support.