Will Original headers fit?
 

I have a 69 Pontiac Lemans with a 350 in it. I removed the engine and put a mid 70s pontiac 400 (6X heads). I left the manifolds on the 400 because i intended to use them but now I want to use the headers. will the original headers from the 69 350 fit on the newer 400?

They should fit. Are the end holes in the heads drilled for the exhaust?

Yeah they should fit as long as you used the same motor mounts nothing should be any different but as suggested check for the header bolt holes.