Yes, 6906 slide should work on a 439 frame.
John
I would like to see pictures when you get if put together.
I'm trying to do the opposite, double stack slide on a single stack frame, but the concern is the same.This is not my area, but a single-stack pistol slide may be too narrow to fit on a double-stack frame. I could see how a later (3906) slide may fit on a 439.
I'll try and remember to do this tomorrow. I have a 4906, a 6906, and a 439.
I have swapped slides on the 5906 and 6906. A long slide compact frame makes a good concealed rig for me.