1974 was the last year the BBC appeared in cars, and you have a '73 tranny so why not a '73.
The open chambered heads didn't appear in production engines until 1971 (they were used in production motors to improve emissions, even though it was sold since 1969 as a performance part in the Chevy parts department). Starting in 1975 Chevy went to the peanut port heads where the port volume feeding a 454 was the same size (190cc) as the high performance SBC head feeding a 350 (regular 350 head is 170 cc).
Peanut ports are great for building torque way down low in the RPM band to pull stumps or 10,000 pound trailers; but rotten for deep breathing required to make lots of power. Since the BBC was only available in three quarter ton or heavier pick-up trucks after 1974 (because they were exempt from emission testing). The BBC's heads were redesigned with the "peanut port" head to compete power wise with diesel engines, that cost three times as much to install in a light truck giving GM a sales advantage.
Under 500 cubes you want a 230 cc oval port head, not the bigger (320cc) rectangular port head. Because under 500 cubes or under 4,600 RPM the motor isn't sucking in enough air to keep the velocity up in the intake runners so gas fell out of suspension. Liquid gas doesn't burn, so you waste fuel and loose power.
Power is made in the heads. You want power buy the best heads you can afford. Match a cam to the power band of the heads (not to make noise) and you will make your power goal and enjoy driving it as well as it will respond to the throttle. A 670 cfm Holley will feed a 454 up to about 5,600 RPM. Above that power falls off and the carb becomes a choke point. If you run a hydraulic roller that is as high as you can spin it anyway before the valves float. With a solid roller you can hit 7,800 RPM before you have to go to exotic parts (triple wound valve springs with 800 inch pound spring rates, half inch push rods to prevent bending, and Jesel roller rockers to stay on top of the valve stem at high RPM).
A 396 is the performance equivalent of a 305 cube SBC. It has a tiny bore that restricts breathing which is why no one uses them.
View from the oil pan looking up a 305 bore to see 1.94" size intake valve. A 2.02 valve will hit the top of the block.
A 396 has to have the top of the bores notched to let the valve pass the top of the block because the bore is so small. Unlike the SBC the valves on a BBC are canted so they open towards the center of the bore as they come down.
a 427 with it's eighth inch bigger bore doesn't need the notches.
The 396 was built as a replacement engine for the 409 because NASCAR had imposed a 427 cube limit on new engine designs (further limited by GM's executive board to 400 cubes in all cars except the full size and the Corvette which had bigger a 427 cube as an option) to reduce the cost to racers who where buying a newer bigger engine every year as the manufactures fought over the win on Sunday sell on Monday advertising war.