I'm not a 261 expert but I think that the pressure port is the rear one (#4) and the return port is the lower one (#3). The 235 is made that way with the return at the rear and at the pan level. Number 2 looks the correct size to be a petcock for draining the block of coolant. I don't know about #1. Are you sure it's an oil hole and not a bolt boss?
BUT..how do you propose to get your exhaust pipe on the manifold with that rear hose on there like that? Looks really close to me and it would be very hot at the least.
Someone here will know for sure after while. In the meantime do a search on oil filters as there is some excellent info posted just recently.
