@oldschoolbob, it depends how easily plug'n'play you want the solution to be.
Knowing you, we're purely talking about Bally/Stern MPU boards right?
The Bally-17/MPU-100 boards have some limitation in the ROM addressing setup so being able to support Bally-35 games requires modifications.
A board that plugs into a single ROM socket won't do the trick, that is unless the board can be modified to run totally from a single ROM socket - I haven't investigated the schematics if it's possible but this solution isn't plug'n'play. If you wanted to use both existing ROM sockets then there is the question of how the sockets are jumpered (9316 vs 2532 vs 2732 ROM, etc)
There is the option to create a daughter board that plugs into the CPU socket where you'd have the CPU and ROM on it, but you also need extra logic to control the ROM. I believe this may have been done before.
Lastly, the solution I've thought about doing in the past is to build a board that plugs onto the upper J5 connector making it the best plug'n'play option - no messing around with MPU board mods but it will require extra logic. An issue with this approach however is the original J5 pin headers are commonly badly tarnished - that pin header needs to be in good condition.