It's configured for '73-up stock Slant-6 carburetors. It's not compatible with '72-down items. The description is misleading:
"Together, the spacer and gaskets are ½ inch thick. This keeps clearance and linkage interference to a minimum"
That's thicker than even the thickest factory carb base gasket. At a bare minimum it's going to require readjusting the accelerator cable and the automatic trans kickdown linkage, which may not have enough adjustment range to compensate. It will definitely cause binding in a pre-'67 rod-type throttle linkage setup. It could also easily cause hood clearance issues (to the air cleaner) on some models.
"Works with any factory slant six one barrel carburetor such as the Carter BBS, Holley 1920, or Holley 1945."
This is not correct. There are no BBSs that will work correctly with this base gasket setup, not even the '73-'74 Dodge truck ones. This base gasket setup will work with 1973 Holley 1920s, but only '73 items.
It would not be impossible (or even difficult) to configure a spacer like this to avoid all height problems and be compatible with '61-up intakes and carburetors ('60 setups would pose an issue of their own, but there aren't many of those out there). With or without that effort put into the product, the seller ought to be more accurate with his eBay descriptions.
That said, studs and (Nylock) nuts with flatwashers are a much, much better way than hex bolts to hold the carburetor to the intake.