Yes, Yes, Yes, the Windsor is lighter that the FE... So what...?
I'm guessing a miata engine is lighter than the Windsor, but you don't see many entertaining that in a cobra. You've got to draw the line somewhere, and some draw it at the FE for an SC...
Now if we were building a FIA or Slabside cobra, then we could talk Windsor...
Else, this discussion is folly
...and the OP will chose whatever tickles his fancy anyway, and happily drive away into the sunset.