How to install engine dipstick tube

[Joemomma1]

During the process of trying different types of exhaust manifolds I had removed the oil tube and dipstick. Unable to find them I went to the wreckers and found one from a mid 70s caprice. It's exactly what I pulled out of mine but I cannot get it to seat all the way down. There is a slight gap between the block and flange of the dipstick tube causing a small leak. It fits snug while pushing it on but it's the last bit that won't go. The lower tube has not been removed. It never leaked with the old tube. I don't want to use silicone or something like that because it wasn't required before but do I have a choice? Is there a way to do this with the engine in the truck?