Hi Aladds,
I dont know how far your project already went and how much extra work this will be, but when you design an new POD PCB, instead of making it fit for Phil's AVR replacement, maybe it could contain an "Arduino nano" (that also runs an Atmega), if it's possible to adapt Phil's software regardingly. So you would need only one custom-made board, and it could be designed to work without modifications in the Control-Box. One disadvantage: Arduino and original Ferranti ULA probably can't share the same socket, so if you want to design it to use either of them, the PCB would need two sockets, and then it could be difficult to find place for all the other components. Maybe some of them could be soldered on the backside, but maybe it gets too complex to design. Well, it's just an idea.
Greetings,
Henning