what calipers do I need????

is there an advantage to mounting the caliper on the front vs. the rear? also steel piston or Phenolic Piston

The factory mounted the calipers to the Front on the 73-76 A body cars.
No advantage, or dis-advantage, to either the front, or the rear.
Do you have a Sway Bar on your 1970?
Guys mount the calipers to the rear, if the sway bar has clearance issues.
If no sway bar, just install everything in the factory position, then you won't have to worry about using different brake hoses.

You will be buying rebuilt calipers anyway, and i guess most likely they will be steel pistoned, but i guess that's going to be dependent on the rebuilder.