Thanks for all of the helpful feedback on the memory addition.
I'm getting tired of building low capacity memory cards. Plus I'm running out of sprawl, so I was already thinking about moving forward a technology revs for this expansion.
I posted in an earlier thread about adding 16k RAM by using 8 * HM6167LP 16k x 1-bit chips. I haven't built that card yet, but I'm thinking that would be a good way to fill out the lower 32k.
For the upper, I wasn't really sure the way to go, but this looks like such a good approach, I'm thinking about doing this with some bits I already have. For the ROM, I'd go with the 28C256 (I have the AT28C256R).
For the RAM, I have a a couple HY62256ALP 32k x 8-bit. I may instead use 2 of these and forget about all of the low capacity stuff I've already done for use with the FDC.
I like the 74HC688 method. I have a signal that is generated back on the first expansion chassis if the memory-mapped I/O is enabled - this was to disable the ROM on the main chassis which isn't fully decoded. This could be used to disable the upper RAM if the memory-mapped I/O has been jumper-enabled.
I'll work out the details and post the results.