dyoungen........according to my hand written notes in my N-Ford Master Parts Catalog (MPC) the float seat is 3/8-27.
You ain't gonna believe this but the eaziest way to remove yer BRASS float seat is to BOIL yer castiron carb part in HOT WATER. Castiron and brass have different coefficients of expansion. You should then be able to pick yer brass threads outta the HOT castiron. Simple, eh? .......HTH, Dell