throughly confused....

Hi to all, I have a 71' demon 4spd build date 4-71 and the motor 1-71. What exhaust manifolds orignally would have come on this.
I see 3418623 and 3418621 as a pair, also 2863549 and 2863553 (Hi-po?) as a pair along with other combos?
Were the high performance ones available for this car and how would you know? Any codes?
The car has 25k on it and I would like to keep it as stock as possible.
The original owner swapped out the carb, intake and exhaust.
PLEASE help, way too many numbers......
THANKS in advance.