There are pros & cons to each location(engine bay or boot).
Most engine bay batteries are located in front of the footwell, where you described. It's just the most out of the way position. Personally I don't think you will see a huge decrease in battery life if it's positioned there...if you are sensible about it.
Make sure the battery is well ventilated, keep an air gap around the sides. Use alumininium heat shields to reflect away any direct heat coming from exhaust etc.
Most people want to keep the engine bay simple and uncluttered, that's why they tuck the battery away.
With battery in the engine bay you also don't need those long lossy cable run from the boot, doesn't take up boot space and the battery is accessible & where most people expect it to be in case of an emergency.
The biggest killer to battery life in my opinion is regular charging. If you only drive your Cobra once every week or so and don't charge your battery in between, you won't get real good life out of it. Definitely recommend a small auto-sensing charger, that you can leave connected.
The late model mustang 302w engines had a plastic fitting in the sump, that could be removed and then weld in a fitting to take the
oil temp sender. You can also buy sump plugs that have
oil temp sender built-in. It's definitely worth getting that
oil temp sender in if you intend to hit the track, the oil temperatures will depress you.