For the most part a small block V8 exhaust manifold will fit any other small block,. the only difference is where it dumps for the pipe to hook to. in general you want to find a pair of maifolds where the driver dumps straight down towards the back of the block, and the passenger dumps at an angle at the back. if youre going to the junkyard, try and cut the pipe after the Y pipe and take it with the manifolds.
if you go headders. about any parts store will sell direct fit headders. you want them for 78-87 G-body, el camino, malibu, regal etc.
as far as your pics, it looks like youre all set.. it has everything minus exhaust to dump in and run. as stated above theres a few things youll need to upgrade, like radiator, trans down the road, different fan shroud (some were V8 shrouds with a clip on spacer for the v6) you may need to move the frame side motor mounts to a different location. the frame is drilled for all applications. you might want to find a V8 g body and see where it is on that car. the hardest part about that is getting to the nuts that hold it down on the inside of the frame.
judging from the pics it looks like an older, possibly 70s motor, two piece rear main seal, driver side dipstick etc.