There are 3 architectures for 60s-70s big-block Chevy engines:
These are general performance passenger car/light-duty truck engines offered in all displacements: 396, 402, 427, 454. They typically have 2-bolt mains, general performance hydraulic lifter camshafts, Quadra-Jet carbs on cast-iron intakes. Some early apps did use Holley carbs; some Corvettes had QJs on aluminum intakes. They are refered to as oval port because the cylinder heads have oval-shaped intake ports. Later 'smog' heads had smaller ports and are nearly worthless. Included are the 325hp/396, 350hp/402, 335hp/427, 360hp/454. Hundreds of thousands were made; intact and running they have value and are in demand largely from restorers, not widely used for racing.
These are special high performance passenger car engines offered in all displacements: 396, 402, 427, 454. Never used in trucks. They had 4-bolt mains, high-performance solid lifter camshafts, Holley carbs on aluminum intakes. They are refered to as rectangular port because the cylinder heads have large rectangular intake ports. Some versions had aluminum heads. Included are the 375hp/396, 375hp/402, 425hp/427, 450hp/454. Fairly limited production; intact and running any one of them is quite valuable because so many were used up racing.
Some truck applications used a cylinder block that had 0.400" taller deck heights to accomodate truck-only 4 ring pistons. This means standard bb intakes and other parts do not interchange. There is a 366 cubic inch version that is absolutely worthless; other versions may have some value to racers. They have no value to restorers.
With the huge selection of aftermarket blocks/heads and Chevy crate engines, rebuilding and modifying a used up engine no longer makes much sense.
Figure out what you have; price accordingly.