[C320-list] Cabling repair without Mast Removal

[Magnuson's Ragtime]

On my, new to me, 1995 - # 205,  The mast was removed and the interior mast cables disconnected last winter for some survey repairs.  This spring the Radar did not work, but due to scheduling, I had to start the 700 mile delivery on Memorial day weekend, so could not wait to find a service tech to determine the problem. I have now confirmed that the problem is the cabling between the Antenna, and the display. In order to avoid the cost of removing the mast and totally retuning the rig, I would like to enlarge the hole at the base of the mast enough to allow the internal connector to pass into the bilge area.  Then replace the cable section to the antenna, and re-terminate the cable ends.   Any suggestions as to how to enlarge the hole without damaging the existing cables? Any suggestions as to hole shape to maintain structural integrity? I am considering drilling second larger hole above the existing and then using a dremel tool to finish the penetration when it gets close, and smoothing the edge burs. 
Thanks,
Gary  # 205