Just to fill in with a picture what Randy said.
Also, does the ring rotate with the lens if you twist the lens? Else, if you can get a firm grip on the lensboard and the lens and loosen it that way you are home.
Then I suggest that you mount the retaining ring, which is actually called a "flange" with screws to your new lensboard, so that you just screw the lens into the flange. (Just make sure that you have marked out the correct position of the screw-holes before you drill them, so that the shutter is orientated the way you want it. (Usually upright.) )
The picture shows the back of the lens and the front of the lensboard.
//Björn