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


as you can see it didn't work
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.
Baz.
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