I installed the D75J TCXO chip last night, and my frequency error went down from -4.65ppm to 0.45ppm, and after warm up and calibration it was down to 0.02ppm and rock steady - a pretty good result, although it was almost "a mod too far".
For anyone else contemplating replacing the existing clock chip with the D75J, please take my advice NOT to try to solder it directly to the board after the old chip has been removed, as the pads on the new chip are tiny, completely hidden from view, and only just coincide with the edges of the existing solder pads, so getting the chip positioned exactly right is a matter of pure chance. I think luck must have been with Insatman when he did it, but I wasted over two hours struggling to get the connections made before giving up. Instead, just cut three pieces of fine wire about 8mm long (34 gauge worked very well) and solder them to pads 4,5, and 8+9 on the chip, then bend them into legs and feet so as to give about 3mm clearance from the board. Soldering the feet to the board then becomes a simple matter because you can now see precisely what you are doing. After the abuse it took I'm surprised the chip still worked in the end, but if it packs up now I know I can replace it without fuss in a matter of minutes!
I think that's all the hardware modding done now, except for painting the box red and putting go-faster stripes down the sides, but that can wait