Any chance the parts are junkyardable near enough you?, the '79 breadloaf 
had the same square ports in the cyl heads.  If it is 'just' your patch that 
has let go can you take the pipe off and get it properly welded up? 
        If it needs rebuilding and the bends in the pipes aren't too tight 
Midas or similiar will bend up a piece of straight pipe for you, you'll have 
to get new flanges made up and welded on, take a new gasket to a machine 
shop and tell them to make you a flange to match out of 3/16" mild steel. 
Total will be way less than $500, if feasible. 
        Usually the bends are way too tight for their tub bending machine 
however. JCWhitneys do sell J-tubes, cheap, perhaps the right amount of 
curved pipe could be sawn out of a J-tube and welded in? I replaced all of 
my '85 wasserboxer system with J-tubes and salvaged pieces plus some plain 
old muffler pipe. No smog checks up here yet!!
   tim s.