55 (and I assume older) 6v point ignition systems do not use a ballast resistor. 56-57 12v systems do. If you converted to Pertronix, no ballast is used. Pertronix I uses a 1.5-ohm coil; Pertronix II uses 0.6-ohm; and Pertronix III uses 0.32-ohm coil.
I recently purchased a Fast PS50 coil (made in the USA) to use with a CRT electronic distributor. The coil is oil-filled. There is an ongoing debate whether to use an oil-filled/epoxy-filled coil, and whether to mount it vertically or horizontally. I intend to mount mine horizontally, in the original location on the intake manifold. I don't know that it matters, but it has been suggested that the coil be rotated so that the terminals are side by side (horizontal) not vertical (one above the other) when mounting an oil-filled coil horizontally.