Are both 120 and 150 mph speedometers interchangable without any other mods ? Has anyone made the change and noticed a difference ?

They are interchangable.

Will not make the car go faster.

Did certain year models have the 120 mph ?

70-72 had 120 except the -6 which had the 150. When the 73's came out, they were all 150 up to the end of 76

My early (11/72) '73 model has 120. It is a 1.7 plain Jane model, so....?