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One prototype 1973 Grand Am station wagon exists. Since it was built from a Le Mans wagon, it is unlikely that it has an "H" in the VIN.
The 455 SD was cancelled in the A-body before any were released to the public. A few 455 SD engineering and press cars were produced in 1973. Supposedly there was one 455 SD powered Grand Am and three 455 SD powered GTOs (two 4-speeds and one automatic). These were supposedly disassembled and destroyed and no "X" code A-bodies are known to exist today. It is not known if these prototypes even had the "X" in the VIN.
Tim Claremont at www.grandamregistry.com has found numerous examples of 1973 "T" code Grand Ams with 400 2bbl engines. It is not known why this occurred. "T" is the correct code for a 400, 4bbl, dual exhaust as verified by various documents from GM.
Only the 400 and 455 were available in the Grand Am.
Dual exhaust was required with a 4bbl engine or the optional performance axle ratio.
Early 1973 Grand Am literature suggests that dual exhaust was standard on the Grand Am. This was not true. Dual exhaust was optional. A dealer service bulletin states that a customer could add it to their car at their own expense if desired.
All 1973-1975 Grand Ams appear to have been built at the Pontiac, Michigan plant.
Starting in 1975 all GM cars had a single catalytic converter, thus there was no true dual exhaust.
Bold indicates the possible and most probable combinations for the Grand Am.