I, too, just purchased a GCII and am thinking through the same issue. I am taking my time with the re-assembly as the table is the finest unrestored original GCII I've ever seen. It was privately owned for more than 40 years, never moved, and re-covered only once. It is still in the original paint scheme (blue/white) which I plan to maintain. The condition of the table is simply astounding - no alterations are planned.
I did a test fit with two of the corner joining rails and it appears the pocket opening is slightly greater than 5.0". I have yet to set up for side pocket measurement. While re-covering the rails before final install, I am going to remove the original cushion facings and replace with 6.4mm neoprene facings @ 60 durometer hardness -
see here:
http://www.pooltablefeltcloth.com/Cushions-Supplies/cushion-facings.shtm
The thickness of these facings will close the pocket opening by 0.25" (versus standard thickness facings) giving a finished opening of slightly less than 5.0". I was told by the fellow who disassembled and moved the table to me that the cushions are in great condition. Accordingly, I would like to avoid the whole issue of sub-rail modification/extension (to fit the presently available K-55 cushion replacements). Thus, for me the pocket facing solution is acceptable. Hopefully, it plays out this way.
I owned a Diamond Pro for 13 years; I'm kinda done with the torture track routine. As many here have stated, let's have some fun playing straight pool!
~Rick