try and warm up the screw with a small torch. apply canning wax and let it cool. it will come out unless someone has cross threaded it.if thats the case (i dont know how handy you are or what tools you have access to) i fixed one by first setting the fuel bowl back on. the i took a drill bit (just small enough to fit down the hole ) that way it will center the hole perfect. then i drilled it out and tapped it to 3/8 because i had a brass 3/8 bolt. i lock tited the bolt and made it flush on both sides. i put the fuel bowl back on ,got myself a new center, drilled and tapped back to the proper size. id show a picture but its running on my 79 ford 460.hope this helps.