Unfortunately that Esci kit is quite a dog... I had one, but when I realized how badly off the body was I got rid of it.
The grille opening and front bumpers are the wrong shape in a major way, as is the side-view/rear roofline...
Since the 250 SWB is one of may favorite Ferrais, I am determined to build one eventually, but hopefully from an Italeri kit, which I hear is much better.
It is, however, quite rare and no reissue in sight, especially since Revell failed to do so last year due to some licencing issues with Italeri - apparently
As for your other questions, I'm no expert, but I don't think undersides were body color, but black with aluminium/silver details, and, although I think there were no factory cars with a tan interior, I have seen pictures of restored cars with a tan/beige interior. Just google it, there's lots online
Good luck with the kit, also... Would like to see it