I am speaking out of turn but my backyard approach would be as follows; get a UNC bolt with the correct dimensions, threadwise- length and shank. Obtain a nut / fitting suiting the pipe fitting. Cut the nut to the correct lenth to suit the pipe fitting depth and trail fit it to fit the bolt head. Tig weld (or have it done) the nut to the bolthead. This would destroy the temper of the bolt. I suppose silversoldering would also work. Put the bolt assembly in the lathe and drill a 1/8" straight through the bolt shank. Machine,or preferably polish the bolt / nut flats to remove evidence of welding. Restore the temper if possible (unless silver soldered) by heating and quenching if really required. Practice extreme care when refitting to the manifold.