I'd probably contact Doug Dutra over at .org for one of his Dutra Dual set-ups -new front 3 casting modified rear 3 exhaust maniflod - let's you keep current exhaust and add new for the front 3. IIRC SlantSixDan has (or had) new slant 2 barrels he was selling - new is hands down preferred over rebuilt (that's a crapshoot!).