I would suggest you start by reading
THIS.
235's made a change in there rocker oiling system in 1958 and again in 1959. 261's seem to have changed slightly later.
To determine which rockers and connecter tube is correct for your engine, you first need to do some checking.
First, does it have a oiling pipe behind the side cover (you will need to pull side cover to know).
Second, if no tube you will need to pull the plug in
Fig #4 and check the orifice size. The description on "Rocker shaft full pressure oiling" in this page is also important to read.
After that I can determine which rockers and connecting tube you need.
