Steve,
1. As Donsz said, you have a 235 head and 235 head gasket, Both have to be drilled to match the holes for a 261 block.
2. The larger thermostat and housing are for a 261; however, you may use either the 261 or 235 thermosta/housing on either a 235 or 261 head.
If you search this forum on "261 holes" you will find more information about the extra holes in the 261 block/head.
Also, look at the 261 thermostat and see if there is a small hole in it. This allows a little fluid to always circulate through the head, even with the thermostat closed. Many people drill a similar hole in a 235 thermostat.
Tim
