Al,
I also bought a GX-60 planning to install in my RV-8, but haven't done so yet. I do have an SL-30 installed, which fits with virtually no room to spare (I had to do some creative routing of the wires).
From the dimensions given in the installation manuals it appears the GX-60 "should" fit:
SL-30 10.775" to back of the unit, 11.451" to back of antenna connectors.
GX-60 10.425" to back of the unit, 11.449" to back of connectors.