A muscle car is defined as a high performance big engine in a intermediate car A Chrysler 300 was not a intermediate!
The olds and Hudson hornet would have fit the bill though so I would say the 1949 Olds was the first Muscle car!
One thing to keep in mind with the power rating of the Rambler the hp of the engine was 288 as installed in the Ambassador
When it was installed in the Rebel it had compression increase from 8.5 to 9.5 and the cam was changed from juice lifters to solid
It don't take much speculation to say that the power had to be up from the Ambassador engine but the rating stayed the same!