Lets not get confused here!
An SPF is a factory built car with very high std's but it is not nearly as accurate as an KMS. Better comparison would be SPF to glass CSX6000. When comparing the body of SPF to a KMS include the correct 427 frame by KMS, floor mounted pedalbox (tough with those long legged folks), trunk accuracy and a host of other internal Ally panel detail. SPF does not have any of this.
This is not a simple Ally body to Glass body comparison. This is really apples and oranges.
Better compare CSX4000 glass to CSX4000 ally. My vote Ally
if you have the means.