There's a reasonably-priced brand new cast iron exhaust manifold that gets advertised on Ebay regularly, intended for 230/250/292 industrial engines, that has the late model 3-bolt flange. I'm not sure if it's for the big truck exhaust pipe or the smaller one designed for pickups, but the 3-bolt flange was generally used with the big pipe.
Search "292 Chevy exhaust manifold"- - - -it's the first listing. $149.00 with free shipping.
Jerry