If doing it on the lathe put some temporary packing against the 4-jaws face for the machined side to rest on, grip in the chuck with the minimum protruding remove packing and take a few light cuts to clean up the other side.
If on the mill use parallels or whatever packing you have to raise the work so it is just above teh top of teh vice jaws and flycut or mill the surface.
Another option if its a GM casting is to soft solder it onto a block of something more substantial to hold by then when done melt it of and a few rubs on some emery cloth laid on a flat surface will clean it up.
J