early bore, square bore and spread bore intakes/carbs

[CLSSY56]

What's the difference in these SBC intakes?

[Tudor]

the types of carbs they take?

[repoguy]

The squarebore is a Holley or Carter type carb with equal size primaries & secondaries, whereas the spreadbore is going to be a q-jet style carb with small primaries & huge secondaries.

[D Picasso]

if you attempt to install a spreadbore carb on a squarebore manifold you will find it to be one giant vacuum leak. the carb's base will oversit the manifold base, and while it tries to run it will likely suck pieces of carb base gasket into the engine. it's a laff riot.