Guys, The stock filter is a Ford staple item....used on early 60's Birds, then 1967 GT 390......then in Mustang/Cougar/Fairlane/Galaxie 68-70 427/428 ram air units, and some "regular" air cleaners. (390's in 68 got a larger unit for some odd reason but used the smaller 428 style in 69....)
FA325 Fram comes to mind....but the mind could be off..... I have one on the shelf....but shelf is 50 miles north....!
Just look for a 1969/1970 428CJ Mustang/Cougar/Fairlane (not 429!) and you'll get the proper filter element!
The stock base is a hand fab unit that DeT made to facilitate this element. It is not a "drop" style base by any stretch of the term. Basically the air cleaner actually lifts the base off the car by a little bit.
When I made my air cleaner out of the 68/69 truck (and MANY others) air cleaner base, it incorporated the stock drop.....and helped me clear the shields due to my use of the Edelbrock RPM intake....which is about an inch taller than a stocker....... I did need to "raise" the clearance on the lid, around the area where the air cleaner bolt comes thru.....
Cheers!