It should start the car as long as there is some charge left in the main battery, I started mine with a 400CCA jumper pack when I flattened mine about 5 years ago.
Deano I have a 55AH Fullriver AGM in the tray now, with 2 heat shields to protect it, I've just changed my battery for my up coming Cape trip, the previous battery was a 60ah Fullriver AGM and was there for 4 years without any problems.
Yes the heat shield is the most important thing, I have 2 on mine now.
This is a recent photo of my second battery, with the second heat shield, the stuff on the shield is paint, that was my attempt at making it look better:whistling::bangin: as you can see it didn't work:mad:
I have since put a bigger gap between the battery and second shield and cleaned it up a bit to improve the look of it.
https://www.aulro.com/afvb/
Baz.