Love that ratty shifter.
How about unscrew the skinny shifter handle, then weld a bolt with the same size threads to the cut off end of the ratty shifter.
With the ratty shifter, maybe unscrew the little aluminum handle, and tap the face of the HBBC shifter and bolt the ratty shifter to it? That really does belong on the build...
One more option, hit up MIGZ and get a Project 346 Shifter from him - you should be able to attach the ratty shifter pretty easily to that one.