Big Bob Evans ran an FIA...and he was a bit, shall we say, tall...former pro B-Ball player now living in Utah. Moved on from the Cobra to messing with Minis. Can't recall just how big Bob was, but he used to rest his feet inthe grill opening.
In any event, if he could make it work, anyone can. The Kirkhams and i used some of his ideas when building mine to fit my girth. Besides removing the seat rails, the use of memory seat foam was a big help (same stuff they use in the mattresses). When you're not sitting in it, it returns to its original shape and fills the seat cover...but compresses flatter when you're in it. Try to get a seat with a suspension on the bottom rather than a hard shell...helps let you sit lower with the memory foam.
Moving the steering column up as high as it will go helps, along with a 15" (even 14") wheel. Moving the pedal box forward, bending the pedals a bit to the left, trimming the bottom of the dash, placing the windshield a bit upright...all are adjustments that can be easily made. Dropped footboxes help as well, but you may be surprised how much room you can get before going there.
Your bigassed feet are always going to cramp your style a bit...hell, my 13EEEs keep me from moving them around on long drives. Besides driving slippers, dedicated racing shoes like Simpsons are great for giving you some comfort. Pilotis tend to be rather thick, as are some other walkable driving shoes. Doesn't take that long to switch shoes at a destination.
As for buying or building, there are some outstanding deals out there, even though FIAs are a tougher find. Hard to pass them up, but building in adjustments for your size may be easier. I went back to Provo several times when mine was being built, and it really paid off.
Good luck! Keep at it and you'll get there.
