You're going to need a custom circuit board. The sockets are so different that a SIMMtree would never fit without an equivalent amount of effort.
It's conceivable that you might be able to gang up 256KB modules, but I doubt it. A cache isn't a simple memory module. It has both fast SRAM for cache storage contents and faster SRAM for Tag RAM which is where it keeps track of which computer RAM addresses are stored in what sections of the cache. I doubt that the tag RAMs gang up elegantly without taking them off the four 256KB DIMMs and mounting them in a new arrangement.
The chips themselves probably cannot be replaced with larger capacity chips, because additional pins would be needed to address the larger capacity--unless there are NC pins on the chips of the 256K version and the circuit board happens to hook them up as needed, and the larger capacity chips needed for 1 MB happen to use those NC pins.
The best thing to do would be to start by tracing out the connections on a cache module to see what goes where. Also get the datasheets for all the SRAM chips on the module so you know the pinouts, as that's part of knowing what connects where. If you don't have a 1 MB cache to examine, the 256MB will reveal most of what you need to know and leave 2 addresss lines as a mystery, most likely. But they may not be address lines, they might be segment selections lines or some such. I don't know that much about cache implementations.
Here is the pinout for the universal firmware module to get you started: http://www.io.com/~trag/Apple_pinouts/Firmware_Module_Pinout.txt Some or many of the NC pins might be used on the cache module. I bet all the data pins and most of the address pins correspond.