I’m usually 100% in the Mopar family but, am currently restoring a ‘66 Fairlane GT 500; which looks remarkably a lot like your Mercury. Best of luck with this fun new venture!
It is currently a small block of some type, could be 289 A-code per the VIN or may be a 302. I have never seen 2 4V carbs on a small block ford of this vintage before so that may be something to check out.
