Back in the early '80s one of the stamping machines for one of the 350F's exhaust pipes was broken (the #2 pipe, I THINK), and since demand for the bike and it's parts was nearly nil, no one bothered to replace it, so nowadays, getting a new 350F factory exhaust is well-nigh impossible.
NOS examples pop up on eBay, but they can command nearly $2000 or more. I nearly bought a snotted out 500F for almost a grand that some guy had stuck new-ish 350F pipes on, clearly not knowing what they were.
AN alternative if you have the downpipes is to get some new silencers from MIVV. They're an Italian firm and it ain't cheap, (but not too expensive either), but it gives you the proper 4-into-4 look and they even look a little better than the factory pipes.
http://www.mivv.it/moto_usa/prodotti_ricambi_case_apri.asp?codice_marche_moto=15