2 amps install and crossover questions

I was planning on using the 1/0 to run from the battery under the truck and up through the floor of the cab

You MUST run the wiring through the firewall...out of the vehicle is a big no-no.

distribution block which will break it down to three 4awg wires

8 gauge will be more than sufficient for any one of those amps, especially using short runs and considering the light loads you will be using.  You'll have far more choices of distro blocks with 8 ga. out.

is there any advantage to using a fused distribution block or would a regular one be fine?

The fuses in the distros are there to protect the outgoing wires, not the amplifiers.  The amps should have their own fuses.  Fuse the wires according to gauge just the way you fuse the main power wire at the battery.

Do you think a 1F cap might be needed with the extra amp?

No, leave that bit of nonsense out of your install.  If you are criticized for omitting a cap, tell the offending party to see me about it  ;)

a nice pre-made 3/0 ground cable

Won't hurt a thing, as long as you have the means to securely crimp a ring terminal to wire that big.  I have an old (100 years!) pair of bolt cutters.  The jaws are all gnarled and pitted and the thing isn't much good for cutting bolts anymore.  But it makes one hell of a crimper for the fat stuff!

should I turn the filters off on the amps or set them also?

I would use the amp settings and leave the deck settings on flat.  You could do either, and as you get this set up you will be able to decide which would work best.  The settings, though, aren't such that you would want to (or allow another driver or passenger to) change them after the system is set up.  You are setting limits to frequencies sent to each pair of speakers and will fine tune them to the Hz.  Someone (or you) fiddles with the deck controls and you have to go back to the beginning.  Set them at the amps and chances are they will remain set.  But keep in mind that only one or the other deck or amp filters should be used, not both.

wire size for the speakers

16 guage will be suitable for all your applications.  The thickest wire you could use would be that which would fit nicely onto or into the speaker terminals.  You don't want to use a fat wire and end up with only 3/4 of it squeezed into a terminal. 

Good luck with it. 

I will surely do that if I ever get up that way, dj!  You hit the lottery with all that valuable wire dropping in, too.  The 150 amp fuse at the battery will be sufficient to allow power to the amps and protects the main power wire.  But when the wire size is reduced (at the distro block), separate fuses must be installed in the smaller wires at the block.  (If you have a fused distro block the fuses you place in the block are for this purpose.)  These fuses protect the individual amp feed wires.  Their purpose is separate from the fuses in the amps themselves.  See http://www.bcae1.com/fuses.htm for a detailed pictorial explanation of why all the power wires have to be fused. 

And that will also explain why it is dangerous to run a car audio system power wire on the outside of the vehicle.  It's just that you want to protect the wire as well as you can and so you wouldn't run it in locations that the engineers of the vehicle would not run high amperage power wiring if they were designing it.  They wouldn't allow such a wire to be exposed to the outside for fear of the possibility of snagging, bruising, etc.  Considering safety of vehicle occupants as priority, route the wire where it will get no contact, rubbing, dangling loops, and all that.  If run under the truck, it should be through a  metal conduit that is clamped to the body securely in several places.