In my '57 Skyliner, 312, the RH rear, just above the starter, freeze/core plug is leaking. Can this be replaced with the engine in the car?

Should I replace all 4 of the plugs, or just when one leaks? Could all the plugs be replaced with the engine in the car?

I'm open to all suggestions. I've never done this before. Thanks in advance.

Tony C in New Braunfels, TX
'57 Skyliner
'56 T-Bird