Hey Dave just a suggestion, but the way I would handle this would be to take a razor saw and cut the ring off the bottom of the Dragon Sherman and put it aside...
I've done a bit more thinking (I know, nothing good can come of that)... Instead of making multiple cuts and gluing styrene disks to resin turrets and making sure that I get the transplanted turret ring glued exactly centered on the grafted styrene disk, I'm again coming back to widening the turret ring on the upper hull.
If I grab some digital calipers, I can determine the diameters of the Tank Workshop turret ring and the ring on the upper hull. I then can easily determine exactly how much the hull ring would need to be enlarged (i.e., by how much I would need to increase the radius of the opening). Again, using digital calipers, I could pencil mark, quite precisely, the necessary increase to the ring diameter and very carefully remove styrene so that the Tank Workshop turret would fit.
To me, this seems far more simple than swapping turret rings. However, as I have only pretty basic building skills, I am open to hearing why this would be a bad idea (in fact, I am very open to learning from those who know much more than I do).
Since I'm taking everyone's time, I've got another question: Why do the Tamiya and Dragon turret rings differ by so much? That is, why aren't the kits manufactured with a turret ring that accurately reflects the real thing (i.e., 69"/35)?