Nothing interchanges between the 235 and the 250. The 194/230/250/292 series was a totally new design using standards from the small block V8. You can take two V8 heads, cut them and weld them together to make a cross-flow head for the newer six, but you also have to drill new head bolt holes. The valve-train is from the big block.
As for dropping a 250 in an older truck, it is relatively easy. The block is narrower, so different mounts and perches are need. For those contact Tom Langdon at
www.stoveboltengineco.com