I had a think, and note that most of the control signals are exactly the same as the Z80. So rather than invent a new board from scratch, I could simply wire the Z180 to the CPU socket of an existing Z80 board (like the SCPUB board),
GND, VCC, D0-7, A0-15, !WAIT, !BUSAK, !BUSRQ, !RESET, !NMI, !HALT, !RFSH, !IORQ, !MREQ, !M1, !WR, !RD are identical, and can wire directly to the Z80 socket.
RTS0, CTS0, DCD0, TXA0, RXA0, CKA0/DREQ0, TXA1, RXA1, CKA1/TEND0, TXS, RXS/CTS1, CKS are serial port signals that can connect to FTDI USB cables or serial port drivers.
A16-A19 are new but uncomplicated, just extra address lines. They would have to go to large memory chips via flying wires.
Z80 CLK of Z80 --> EXTAL
PHI is the CPU clock output.
Z80 INT --> INT0
INT1, INT2 are extra interrupt lines, not needed yet.
DREQ1, TEND1 are for DMA which I can ignore for now.
E is for Motorola peripherals which I don't have.
ST works with !HALT and !M1 to indicate CPU state. New states are DMA, HALT, and SLEEP.
XTAL (crystal drive) is not used because CLK is driven.
This doesn't look too difficult to deal with.
Transplanting the Z180 into the existing SCPUB board could be done without having to re-write the serial port drivers. There is no reason why the existing SCC chip must be replaced by the Z180 serial ports. The BASIC interpreter ROM can continue to use the SCC.
For a completely new board, one would want to do without the SCC for reasons of economy but I shan't be doing that just yet.