Hi Deni, I bought a pair of MGB seats to put in my Spyder and they turned out to be too big. You might struggle to get them into your Miglia as the bodies are somewhat similar in size I believe.
The problem is the width when you lower them enough to avoid the dreaded 'Noddy' driving position.
It looked like they'd be fine on first trial fit -
Still looked okay with the floorpan dropped to lower them -
But then you add the frame and, oh dear, they don't fit -
Don't be tempted to fit them above the chassis rails (as in the first pic above), you really won't enjoy the car if your seating position is too high.
It would be very uncomfortable and your head will ruin the aerodynamics.
Ideally you want your eyes to be level with the top of the flyscreen and your shoulders to be just above the edge of the cockpit so you can drop your elbow over the side while you're out cruising.
Anyway, good luck with your build.