angle grinders are the shit.. work well on the plastic too.. just be careful.. because it will literally melt the plastic (which gives nice smooth edges = good), but if you get any of it on your hands it burns it burns.. got a couple of nasty blisters from being stupid. The other option that will work fairly well is a jigsaw. I used that at points too.
You can pretty much get a result that looks like it was meant to be, if you are careful. I only used the file to shave a few bits out a bit further, but the file can tend to burr the edges.
Chopping that support with the angle grinder is pure entertainment in itself.. why pay somebody else for the pleasure??
Install took me about 5-6 hours for cooler itself, but a bit of that was trying to workout how to get the bumper and bumper skin off.. work out what to drill, what to cut, etc, etc.
For the piping, if you purchase the required bends, some straight pipe, and dummy fit everything.. if you mark a line across both pipes to determine which angles they should sit, you can take them to somewhere and just say "weld this here" and you should get a cheaper result. That was what I was going to do, but in the end, I ended up buying a 2ndhand kit off another member for a good price.