I remember reading in the forum about what a pain it was fitting the new pups to the old pick-up cavities. I so want to do that to my 5120.
That was probably me back a couple of years ago when I upgraded my 5120 so that it's basically a 6120 now. The big pain in doing all that upgrade is that the 5120 was setup for regular humbuckers that hang off the pickup ring. TV Jones sells a set that will work with those, but I heard a set of his Ray Butts Filtertrons, and figured that, if I was spending TV Jones money on pups, I wasn't going to be happy without those, and they didn't make those in the non-traditional style. So I had to go into the guitar and add figure out how to add the bracing that those nicer Gretches have so that I could put in the nicer pups.
The regular TV Jones pups were like $130 or so a piece, and the TV Jones Ray Butts pups were like $175 a piece. I wanted to get some cheaper pups to be sure that I had all the work done right on the guitar before I spent that much on pickups. So I ended up going with just a regular set of Gretsch Filtertrons that the base 6120s were getting at the time. I ended up getting a pair of those for the price of a single TV Jones Classic. Ends up that I've been really happy with them...super clear and articulate. I've liked them enough that I can't justify spending the money on the TV Jones pups now. I bet the TV Jones probably sound better, and I know the Ray Butts set sound better, but, at this point, it's like the difference in spending $100k on a 'Vette or $300k on a Ferrari. I'm sure the more expensive ones are better, but the dollar to performance ratio on the graph is a slightly perceptible slope rather than a hockey stick. The hockey stick on the graph was the difference between the stock 5120 Gretschbuckers and the Filtertrons.
Long story short: That was probably me when I turned my 5120 into a 6120, but ended up using regular Gretsch Filtertrons instead of the TV Jones.