Are they smaller in diameter, smaller in length, not as many coils? I know the tensil strength is not as much as metal hoods.
Which year Shelby Fiberglass hood are you asking about?
I don't think (?) the '67 Shelby fiberglass hood spring(s) is/are the same as the earlier year(s).
See attached below - the "smaller" on the right side of the image is the ALL fiberglass hood spring for '67's which is not what is shown in the top position of the above posted image. Note the ends are different.
The spring on the left side below is the stock Mustang spring (maybe the same as the lower spring in the image posted above) used on the early fiberglass bonded to the metal frame of early '67 Shelby's.