Squarebore carb to spreadbore manifold adapters are something to be avoided except as a last resort. None are good, the least poor of them being the one Mercruiser made and used on GM V8, but most are terrible. Almost all squarebore 4bbls will have larger Primaires than a spreadbore carb, and much smaller Secondaries. An adapter will, on the primary side, be restricting the flow from the larger primary carb bore. This will result in differences in vacuum levels and velocities in the intake Compared to what is below the carb in the adapter. While on running Where only the primaries are open, it is similar to a carb that is too big for the engine. When the secondaries start opening, they are facing the much larger secondary holes, and will see the intake vacuum with no restriction choke down. This will cause some imbalance between primary and secondary sides and a reduced signal in the primaries, with a possible richening of the gas-air mix. The secondaries will be getting a stronger signal, that may lean it out compared to the primaires. An equalibium will establish itself but it will far from ideal, when compared to a proper carb to manifold match
