With your new information in mind, I checked Rockauto.com for "thermostat housing gaskets" for both the 1957 235 I6 and 283 V8.
They list the following T-stat gasket brands and part numbers that are the same for both engines:
Mahle # C24109
Felpro # 35062
Would appear the gaskets are the same, so good possibility your V8 riser
may have the same bolt size/placement as the 235 T-stat housing.
An easy step to confirm bolt size/pattern spacing would be to take your 235 T-stat housing to your local Flaps and compare it to a V8 housing to confirm dimensions.
Would be interested to hear about your findings.
