FE (Forf Edsel) is the original Big block motors the CSX3XXX cars got (What you think of as a 427 Cobra) many actually got 428 motors as the 427 was more expensive and harder to source when Shelby was manufacturing them. The 428 had a smaller bore and longer stroke than the 427. The 427 motor is what's known as the "Side Oiler" as it had a different
oil galleries and priority main lubrication and no provision for hydraulic lifters.
Do a bit of reading on this forum and you will learn everything you ever wanted to know about the original Cobras.
The Boss motor we are talking about is the new Modular 5.4 fitted to the XR8 Falcon. There were a couple of Ford motors to wear the Boss label over the years. The original being the Boss 302 which was efectively a 302 Windsor bottom end fitted with the big 4V cleveland heads. This was a screamer motor built for Trans Am racing in the US. Later there was a 351 version in the Mustang but I think this was just a warmed up 351 windsor. The mightiest motor to wear the Boss label was the Boss 429 used in some early 70's mustangs. This was the 385 series bottom end (what is more commonly seen as a 460 cube motor). It had some rather special hemispherical chambered heads and was an absolute weapon. It was sometimes called the Shotgun motor. It was built by Ford in an attempt to unseat the mighty 426 Chrysler Hemi.
If we could run the original boss motor I would have one in my Cobra. I'd build a tall deck Windsor bottom end with a set of CHI 3V Cleveland heads on it and make a lazy 600HP.
As for comparing the size of the size of the LS series engines to the FE? They are about the same size as a 302 Windsor but with a much more compact intake setup. All alloy construction also makes them significantly lighter than the Windsor. Pretty amazing for a motor that can be punched out to more than 454 cubes using the Bowtie LSX block.
I love my Fords and have owned quite a few but I just love the engineering in this LS series motor.
Cheers