Interesting project MarkS. By coincidence, I've been reading the following similar projects in the last 24hrs:

Prop-6502 laptop

pPropQL020 and pPropQL

I think the "16MHz" quoted for the Plus is probably nominal (ie near enough) and is actually 15.6672MHz.

Have you considered taking your clock straight from the Mac's onboard oscillator? That way you know the two systems are in lockstep.

Is there some way of locking two separate oscillators together? Say, the Mac's at 15.6672 and yours at 62.668 (4x), if such an osc is available. Or dividing yours down by 4 and driving the Mac's osc socket from that signal?

If you can't find an exact match, you could try this adjustable osc:

LTC6903 1kHz-68MHz Serial Port Programmable Oscillator