If you are going 60's vibe, the iron 409 heads all the way... although, I would probably throw on a set of 340hp heads and sell the 425 heads, worth quite a penny. (Especially if the date codes match up with an impala SS owner with the correct build date)
Oh, and since I'm giving my opinion...
A flat head would be my first choice, preferably a merc with fenton heads and two strombergs.
Second choice would be a 265-283ci SBC, 097 Duntov cam, power pac heads, three dueces on vintage offy intake.
Third would be a small early hemi