The SD455 head is truly the best head from a performance standpoint, but the huge chamber on the head, required to maintain low compression, was a killer.
On the topic of port comparisons and port flow, the SD455 stands alone. The Pontiac Engineers hogged the SD455 head intake runner out so much that they broke through the pushrod bulge (the main airflow obstruction on a Pontiac head) within the runners. They remedied this by inserting special pushrod tubes in those runners. They also raised the port roof so high that they broke through the valve cover bolt holes. Tuners soon found out about this, as many Pontiac people replaced the factory rocker arm lock nuts with adjustable versions for a few more rpms on the SD455. With the engine idling, setting the adjustable polylocks proved to be difficult. The engine is idling, the valve cover is off, and you're wondering why the engine is dying? On the SD455 cars, removing the valve cover bolts over those runners caused a massive vacuum leak.
You had to replace the necessary bolts if you idled the car with the valve covers off.
Seems a little extreme for 1973, but the SD455 was the Pontiac bad boy that made it to the party too late.
Pete McCarthy did some serious flow work tests a while ago, and ran all the Pontiac heads through a Superflow. There was always some debate on what was truly the best street head.
Best performance head : 1970 Ram Air IV
Best intake port : 1969-70 Ram Air V
Best exhaust port : 1968 1/2 Ram Air II
Best D-port head : No.16, No.48, No.12 (tie)
Best low-compression D-port head : No.96(1971)
Best low-compression post-1972 head : No. 6X
Best balanced head (exhaust to intake) : 1963 421 SD
Best low-lift (under .400) head : 1967 No.670
Worst exhaust-to-intake port ratio : 1969-70 Ram Air V
Worst intake-to exhaust ratio : 1968 1/2 Ram Air II
Biggest surprise : Intake port, No.17 350 head
Biggest disappointment : 1969-70 RA V, 1973-74 455 SD (tie)
Biggest "sleeper" : 1975 No. 5C
Most undercammed : 1963 SD, 1971 455 HO, 1973-74 455 SD (tie)
Most underexhausted : 1964 GTO (No.9770716)
Most potential for porting : 1973-74 455 SD, 1968 1/2 RA II (tie)