I thought distributors for the GMC 270/302 and Chevy 235/261 engines were the same. I contacted a company that makes a mini HEI for the Chevy 235 and they said it would not fit my GMC 302. Can any one comment the distributor interchangeability between the late 1950's GMC and Chevy 6 cylinder engines?

They are the same. The newer generation 194-292 has a little shorter shaft but with a spacer the old distributer can be used in them but not the other way around.
I have used Mallory unilites and Petronics for years and recently went back to points. Points have never left me on the side of the road. Well not for long.

Pay attention to the distributor gear being used. If you are running a cast cam or a steel cam, you need to run a compatible distributor gear so as to not destroy the drive gear on your cam.