trouble adding oil to 260 V8

I am working on a 64 1/2 Mustang with original 260 in it. I tried to add oil to the filler tube which is located in the timing cover. Oil won't go in! Engine was overhauled years ago by someone else and car hasn't run much sense. Is there a gasket or something that might have been put on wrong?

Any 260 experts out there!!

Thanks,

Dan