The FL head will fit but there is a problem. The valve springs were far stronger than the Major and the FL tractors had uprated cam followers, cam shaft with a single bolt fixing, timing gears as well as the rockers. Even with all the upgrades Supers with FL heads would eat cam followers and cam shafts back in the day. Our dealership changed a lot on relatively new tractors.
One of the first questions I answered on this board twenty years ago was to identify "rings" found in a Super Major 5000 sump. They were from cam followers where the centre had been worn out. I got a new set and a new camshaft and sent over to the US and it was not cheap.
I would stick with what you have.