As for the relocation, it cleans up the engine comp't and gives you better weight distribution and traction to some degree. I routed the cable inside the car to the trunk in my fastback. Left the solenoid in engine bay.
Sealed or not, you'll want some way to vent the battery outside the trunk. A lot of people use sealed boxes and then the box has a vent to underneath the car somewhere. NHRA rules require such a box if a car to be raced doesn't have solid steel "firewall" betweent he trunk and passenger area. Mustangs don't, unless you've added one behind the seat. Even if you never intend to race, keep in mind that almost all the NHRA's "rules" are intended to keep people safe. No harm in following them.
If you don't want a box, you can just secure the battery and find some vented top caps. Original Shelby's had these, they had a hose that ran through the trunk floor to vent outside the car. Some BMWs, Miatas, and 90's Olds Auroras used the same setup.
A third choice would be to use a "dry cell" or truly "sealed" battery. An "Optima" gel cell is good. They aren't truly "sealed" but don't vent enough to be a problem.
All this venting stuff is about two things. A battery in use puts out some nasty gasses. One is flammable. Imagine a badly gassing (overcharging) battery in a sealed trunk and someone opens the trunk with a lit cigarette in hand. Nasty.
Other gasses are corrosive. You might see some odd long term corrosion in the trunk with an unsealed/non-vented battery in there. Picture the white gunk on your old battery terminals growing on your trunk hinges.
And yes, my 67 coupe's battery is in the trunk. And vented. ::