http://www.accurateltd.com/Flanges_c_10.html

I had a similar leak, what Tom from Accurate suggested & I did with good results was bolt the headpipe to the manifold with the new flange sandwiched between the old flange & the headpipe... Tack the two flanges together then drop the pipe down & fully weld it.. Winds up with a 1/2" thick flange & no more leaks...