Squareback vs round back alternator?

I think that ballast resistor will work OK. make sure it measures what they claim.
From the 1960 Dodge shop manual
upload_2022-3-17_11-48-26.png

1965 Plymouth Shop manual
upload_2022-3-17_11-52-8.png

So .5 to .6 Ohms is probably correct match.

The older orignal ballast resistors are made with a metal bracket for mounting. Othewise the shape is same.