A quick update to note that I've wired up the generator G outputs on the model 74181. I re-organized the board a bit and co-located them with the propagation bit logic.
The board is a bit less clean now, but I think the trade-off is acceptable. Running wires to the P & G intermediate output LEDs creates a bit of a stacked bus in the middle of the board. If I didn't have those toy LEDs the overall layout would be pretty clean. It also doesn't help that b and s0 is as far away from where it's used as it can possibly be, resulting in the long white wires on the bottom of the circuit.
I've run through several test cases and everything is working, although I won't be able to show this well until I get new DIP switches. The current ones like to pop out of the board so I have to hold them down while testing.
Now I just need to add circuitry for calculating the sums and carries and then I'll have a completed ALU!