I'm planning to install the U1MB in my 600xl and in checking my 64K memory upgrade, I found that it does not match any of the common upgrades. I put it in at least 15 years ago, maybe more and I don't remember where I got it. The 64K chips have their markings sanded off. I have wires soldered to the lifted legs of U5-3 and U6-9, but they go to U2-4, and 5. Also U18 pins 8, 9, and 10 are lifted. There are just the two wires. Does anybody know anything about this upgrade?
Sounds ok. The lifted pin at U6 should be 10, not 9. The both cables connect the missing address lines needed for selecting something over 16K - A14 and A15. You can take these both from the MMU like in your solution, the CPU, the 74LS375 (U15) or some solder joints.
U18 (74LS32) is only needed to limit the CAS signal to the lower 16K. When pins 8, 9 and 10 are bended up, there must be a connection between pin 8 and 10 somewhere, maybe from the bottom of the PCB or chip itself.
I see no problem to install an U1MB. Remove the both wires connected to the MMU and solder them to an other place, for example:
A14 = pin 9 of U15 (74LS375)
A15 = pin 15 of U15 (74LS375)