Can someone tell me if a stock seat from an 07 FXSTC Custom will fit on an 03 Softail Standard?

 

Nope, It will not fit a stock tire/fender setup.
The Custom's seat is made for a bike with a wider rear fender (200mm tire).
It also requires the sissy bar to bolt it down and that is also made for the wider fender.

 

Well that takes care of that! Thanks!

 

You "could" put one on. But it might be more trouble than its worth because you would have to change out some things in the rear to accomodate the parts that get you the 200mm tire and fender combination. Like the belt drive, fender, wheel/tire, tailight, fender support rails, and maybe the swingarm.
"Exactly" what might be involved is something that I'm notsure of yet. Mostly because I personally don't know if the frame is the same and they just changed the suspension/blet driveparts in 06 when they introduced the 200mm tire, or they redid the rear of the frame too.
You might check with a couple of the seat makers and ask them if they can duplicate one made to fit the older narrower fender. Never know, somebody might do it for cheap just to use your bike as a mockup tool. Ya never know until you try.