A 390 will ruin the car. If you want a "Holy Shart" machine, drop in a 351-C or a 351W. With aluminum heads, you can get the weight down, and with the right block, you can keep the C-4.
When I were a wee lad, there was a pretty nice looking babe who cruised our local strip in Daddy's 351W Mustang II FB, and almost never lost to anything -- including some righteous BB's. The swap was clean as a whistle, and most of the strokes believed the 302-2V sticker on the air cleaner... or at least they did until they got their ass whipped. And that was when the most exotic speed parts available were the C9OE heads and Hooker headers!
The FE is great for an old T-Bird or a Galaxie or an Efffie, but a waste in a teeny car like the 'Stang.