Those are later LA 318 and appear to be for a car as opposed to a truck. There may be a P/N stamped on the flange. If you need headers for a poly keep looking and be prepared to part with more US Gov't issued fun tokens.
Satellite66; yeah they are pricey & not very many available "I think" here's some from Hooker Headers/Super Comp for like $633 on E-Scam Motors... you need the flange with evenly space tubes, not like the one you posted that are for latter LA or Magnum 318ci with a siemiezed together type of ports on the center 2 tubes... if you understand what I mean... you may try to call Schumacher Creative services tell them what you have & need they "might be able to help", they won't be cheap thou.... I found those on E-Bay under 318 Poly Engine, I think it was... there is a lake header kit also to build your own but probably not what you looken' for...
Unfortunately these won't work either, if they are as pictured. Poly exhaust ports have bolt holes on top and bottom of each...not at the sides.