Any drawback to keeping the drum spindles on my 66 Valiant?

With the taper that the Wilwoods have that might work. But keep in mind that’s the same as reducing the backspace on the wheel, it will limit your tire width. Basically it would make the cop wheels a 15x7” with a 4” backspace instead of 4.25”, which would mean you’d be maxed out with a 215.
Yes, absolutely true - but isn't is still less width than the 73-76 disk spindle adds? In any case, I'd have to mock-up the hub and wheel to confirm whether the wheel would seat onto a 0.20-0.25" spacer.

Thanks for bouncing these ideas back and forth!