Does Series III Neck Fit Mark VI ?

I'm sure you can get it to fit by taking it to a repair shop; they can expand or contract the tenon as needed. MK VI necks don't have a standard tenon size as the new necks do. BTW, I tried a Ref 36 neck on my VI a couple of years ago and it did nothing for me. But the III neck is supposedly louder and brighter, right?

You may want to try a neck with a different angle, not the same as a MK VI. For example, MK VII and Ponzol necks have a "higher" angle, (than a MK Vi neck), and, if you find one that works for you, can make the horn more free blowing, (with less resistance).