From the things I learned in Automotive electronics class is basically telling me to B!tch slap you for wanting to put a bigger fuse in the box
I always recommend against it. LEDs pull a trickle of power, and if you do blow a fuse run a new dedicated pwer wire.
The fuses in automobiles are selected for a reason (basically to not over power a wire, Always remember fuses are chosen based on the gauge wire, not the item there trying to power...). Think 20 years down the road when 3 or 4 people may have owned the truck, each one bumping up the fuse amperage for something or other, now someone has a nice truck on fire due to over powering a circuit.
Basically is what Im saying is you should be fine just tapping into the existing line, no fuse upgrade needed. ESPECIALY if your replacing the existing load, not using it with the new load (ie pulling the old bulbs out, then using only LEDS)