No way any FE will go into a Maverick w/o serious mods. The Tunnel Port equipped car you saw likely underwent the knife to seriously cut back the tops of the spring towers, probably all the way down to the horizontal frame member. This would look like the 69-70 Boss 429 mods that Ford made whereby the towers were cut back, along with a few other mods.
Hey, I looked to do the same many years ago (but with a SOHC 427, ala' Dyno Don's Maverick) but decided all the work wasn't worth the effort w/o a full tube chassis.
http://www.gilchristbnw.com/GPC/PS-DDNGallery293.html
Today, best bet would be to buy and install one of those aftermarket front clips using Mustang II components. Then, any FE would easily fit although the headers may still need to be custom. Some off-the-shelf headers might fit but how does one "borrow" new headers to see if they would fit?
http://www.cachassisworks.com/iwwidb.pvx?;multi_item_submit
