I didn't know that, but from the description the PHF and PHM take the same float bowl. FYI - the PHF carbs are 30 - 36mm, PHM are the larger size 38-41mm.
The early carbs used a smaller float bowl nut that tended to strip, so at some point was increased in diameter. That appears to be the only difference - the size of the opening for the retaining bolt.
There's gotta be a way to clean out that gunk - but I know what you mean. I've got a gas tank that's half full with the same shit - It looks like it got filled with hot tar. I may have to have a radiator shop clean it out - the paint is shot, so no worries there. So that's a possibility for you - see if a radiator shop will clean them up for you. If you decide not to mess with them, I'd be interested in getting them as an experiment.