While not a simple job it's also not a wretched one either. 5/16 is too small and I'd go with a low stretch line, maybe spectra, but check the specs on the spool to get the lowest stretch. If in doubt take a piece of the old line when getting a replacement and buy more than you'd think you'll need.
On our 2004 260 support the boat raised at least 16" off of the CB at the leading edge and slowly release the CB line. When the bolt in the cabin well is removed you should be able to wrestle/wiggle the SS CB assy down to access the line connection on the CB. It weighs about 80lb so be careful. Be ready to deal with worn/missing parts at the CB head. I was lucky.
If there is a single thing that'll make life easier in reassembly it'd be a 16" piece of all thread rod the same size as the main bolt (3/4"?) screwed into the CB bracket to guide it back to the hole inside. Lastly putty it all up with compound of your choice being careful that overflow won't foul the line and pull/push it up into place. Cover the bolt head in sealant after it's well tightened.
A few pics of our experience including the shocking condition of the main mounting bolt that was only 6yrs old.
Good luck. mike