Will full length headers from a 72 olds 350 engine fit on a 70 455 engine?
Will the exhaust manifolds interchange as well?

 

yes and no. they will bolt to the engine but wont fit in the car. the 455 is taller and wider than the 350 so they bend differently to fit car.

 

Actually no and no. You might be able to bolt them to the block with it on an engine stand, but you'll never bolt of a starter or the oil filter adapter.
Due to the taller deck height of the 455 than the 350 the tubes need to be longer to clear not only the floorboards, but parts of the engine and the steering column as well.

The SBO exhaust manifolds will bolt up just fine (been there, done that) though, but I don't really recommend that due to the flow restrictions that they cause.