Eprom technology was hot stuff in the 70's and 80's, most of us have moved on since them...
Use a flash based PIC or AVR instead - much easier once you know your way around them... for example, it you want channels 41 to 80 with an EPROM, you'll need the codes for input (switch) and output (PLL) - that's 80 codes all up, just for an extra 40 channels.
With a PIC/AVR, just program <original code>+45. One line of code 
Other advantages:
1) No need for ultraviolet lights to erase them
2) Can be programmed serially in-circuit
3) Can do other tricks, like:
Scanning/searching/priority or call channel monitoring (dual watch)
Can drive serial PLLs for thousands of channels
No need for lots of switches to select bands / shifts (one extra switch is all you need)
Much easier to obtain parts
Much cheaper (a chip you could do a 1000 channel mod with costs a dollar or 2)
Not limited to 40 channels per "zone" (i.e. you could have one or your ranges as 00 to 99 covering all of 28 Mhz)
We did a Stalker 9 SSB rig with one of these, and a serial PLL scrounged from a 90's era Nokia mobile phone.
It covers 25.000 to 29.999 continuous in 1K, 5K, 10K, or 100K steps. Radio looks original (no extra switches) except that it shows the frequency in freeband mode (display shows Mhz on TX and the first 2 Khz digits on RX.. ie international call shows "55' on receive and "27" on transmit).
Total cost of parts used was under $10.
