I don't know about fitting in to FIAs, but I did take the time to fly up to ERA and visit with Doug for a true butt-in-seat test on my 427 car. Peter was doubtful that Doug could fit all 6'4" of me in to the car comfortably -- and I told them that I wanted to look squarely through the windshield, as if I was 5'10". Doug said he could do it... and he did. The driver's seat is moved back, bolted directly to the floor, and has reduced padding. The pedals are moved forward with a special Tilton clutch master cylinder -- slight additional effort, but if you're 6'4" you don't really care. The pedals are spread a bit to accomodate my 12 EEE feet (pics in my gallery along with a tape measure). 15" Moto-Lita wheel and I'm good to go. With my top up (on the very, very rare occasions that I drive with the top up) I have more headroom in that car than I do in my full size GM car.
It can be done. There is a small downside though, when Danny D. brought my car down to me he crawled behind the wheel to back it out of the trailer and exclaimed "My feet won't even touch the pedals -- and there's no adjustments on anything!" So I had to back it out for him.
BTW, Doug told me that he had to craft a sort of "baby seat" to sit in the driver's seat itself so that he could then drive the car. Now that's funny.