Just dug out the pictures of the MGB GT seats I tried in my Spyder (hope they appear, photobucket are being a bit of a pain, these days...) -
Fitted quite snugly into the chassis once the floors were lowered enough to avoid the 'Noddy' driving position -
Very comfortable and a nice, laid back driving position -
But, once the frame went on -
The upper frame box-section tube is only about 2mm outboard of the inner cockpit edge and the Miglia and Spyder bodies are virtually identical (in case you don't know, the Miglia prototype was just a Spyder modified by Gary J's ex assistant) so you might well face the same issue. It would've taken major seat frame surgery to the tune of about three inches narrower to get them to sit right (no pun intended), but they'll be very comfortable if you can use them.
Good luck getting them to fit.