I was taught the hex were for chasing and round for cutting.
Interesting, an old wives tail then.
Hex Dies are only for re-threading not for cutting threads....
Round dies are made for cutting threads....
I did something very similar. To make a hex die adapter, I started with a $7.00 die stock off Ebay, sawed the handles off, and turned it round. Then I made a collar to fit my 1.5" round die holder and press fit the dismembered die stock into the collar.I made a Hex die holder from a large socket that I cut down and inserted into the die body. Little bit of Loctite and it's worked well. If the Loctite fails I'll drill and tap for a set screw.