I run a '92 T10 4.3 in an  '80 frame.  Stock exhaust manifolds would not fit, they hit the top of the frame.  Found stock tubular style manifolds from 80's something 4.3 at the junkyard and still had to cut and bend the collectors to get them inside the frame rails.  1st gen frames are narrower than 2nd/3rd gen.  If you have the cash get the Sanderson Headers.  Blingin' good stuff!