If you look at later model 302 and 351W oil pans, 80's, 90's, that have flat side rails. Older oil pans like these classic cars use have ribs between each bolt hole on the side rails. Sometimes somebody will try to install a late model one piece gasket for flat side rails on an older oil pan with ribbed side rails. It will fit, but might leak.
If you installed it 2 years ago and now a leak has started. Look close and try to determine if its the oil pan gasket or rear main seal. It's not too easy on Fords because of the plate between the engine and trans. Silicone sealer is kind of a strange thing. It's for sealing oil leaks, yet, oil slowly deteriorates it.