Wow where to start! First, Welcome to the Alembic Club.

I've waited for this thread for a long time, hoping it'd be mine to start one day having been a close follower of McVie's for... forever I guess. Fair to say I might not be here if not for him and those Alembic basses. Some of the DNA from his '76 "Continuously Fretted" Custom Series I with its stainless steel fingerboard went into my own Custom fretless a few years ago.
Like you, he's played about everything with four strings (and a couple fivers too) but all four of his Alembics (that we know about) were Series basses. You specified the Rumours-era band as reunited for The Dance tour though, and by that time I don't believe John was playing the Alembics out anymore. I haven't watched it in a while now, but that MTV concert, the video at least is mostly his maple Tobias, a couple tunes with a Turner Renaissance fretless, and a Fender Precision bass. I think most of the audio that is post-production overdub, and anybody's guess what he actually played. (I could swear a couple tunes are fretless, that he's filmed playing the fretted Tobias) I don't have any trouble believing he played the Tobias though, it clearly was his main axe at the time.
What Alembic for that gig of yours? First - Yes, I think it matters. I think it will matter to you, and anyone who knows what those songs sound like. You already know this part; John McVie turned being invisible into a persona... he doesn't like to be noticed. But those mega-hits don't sound right at all without his bass lines. Can you get there with less than a Series bass, I think so. I have spent hours upon hours learning every track on the records that culminated into that reunion show. If I were looking for an Alembic bass to approximate the Tobias bass sound from that era of Fleetwood Mac, I'd be looking for a Europa at the upper side, or even an older Distillate. A Spoiler could get you there too. Be advised, most Spoiler/Persuaders and Distillates are medium-scale (32") if that's a deal-breaker for you. On the lower end, a maple Epic has the 2-band eq and a more familiar long-scale. Aesthetically close too.
You all sound great in the promo too. Best of luck with the tribute. Wish I could get something like that going here.