Don't have a 70 MPC and the later versions only show replacement springs and only lists "Standard" & "Heavy Duty springs". Of those there are 6 different springs listed and applications are only 302 2V & 351's
Likely the spring rate is higher and the final height different.
Example (hope the MPC listed them years later)
Standard spring with no options on a 351 convertible - just for comparison
Standard = 1520 lbs., .600 wire dia and 16 1/4 "
Heavy Duty = 1500 lbs., .625 wire dia and a height of 14 3/4"
On place to find an exact replacement would be from a donor car with the same options and body style from the same time period since it appears that the front springs changed early in the production year