What is the function of the bleeder screw? To remove the air when bleeding, and it can only do that when it is on the top.
The good news is these can work just fine. To bleed the one with the screw on the bottom, remove caliper, insert a block of wood between the pads, flip the caliper 180 degrees so the bleeder is on top. Now bleed the air out. You will need a helper to do this. Remove the wood block, Compress the piston back in and install the caliper.
The bleeder screw can be on the bottom when driving. It’s not a problem. I have a car operating like this right now.
The original 66-69 B-body calipers were Bendix 4 piston. They aren’t worth having unless you want the car 100% correct. They use a different spindle and rotor.
When the calper is off take your pads to the store and match them up. Or match them up to a friends pads. Then you know what they fit.