Nick... I'm probably not the best one to advise you, but I can point you to some projects from our members that will help get you started.
One of the first projects I followed was
Warthog5's X-Shark project over on ClassicMako.com before ClassicParker was even born.
The post is still over there, and here is the link:
http://www.classicmako.com/projects/xshark/
Next would be Bobby's 2530 project here on CP: viewtopic.php?f=11&t=12419
Here is another 25 refurb by member
GoodChance: viewtopic.php?f=11&t=9962
A little closer to your 18 project is this 21 refurb by member
windknotnc:
viewtopic.php?f=11&t=10079
Each of those major projects contain tools, tips, and techniques you need to employ in order to make your project a success.
What you are looking to do is much simpler than the projects above, but before you begin cutting, you might want to see what is involved.
As always, the search tool in the Projects section is your friend.
Having said all of that, this is what I would do:
1.) If your plan is to add a below deck fuel tank to your boat, you are going to have to cut the deck at least enough to expose the area where you want to install the tank.
I would center it in the hull if practical.
2.) Since you need to cut the deck for the tank anyway, that would be the perfect time to install a proper rigging tube.
If it were me, I would run it from the box were the cables are now, except I would fabricate a PVC tube from inside that box forward.
Since you will need to take that rigging tube over to the re-located console, you will need to go through a stringer (or two).
This is where you want to look at the other projects closely to see how that is done so as not to comprimise the integrity of the stringer(s).
When you fabricate that tube make it as large in diameter as practical because you are certain to want to run something through there in the future.
I have never see PVC for sale with a radius like you see metal pipe, Though if you can find some, that would be ideal for your transitions.
Otherwise go with 30 or 45 degree elbows to make the transitions as gentle as possible. You want to keep snags to an absolute minimum.
Make sure that the rigging tube comes above the deck floor inside that rear box to keep water out of the hull.
Do the same inside the console.
Finally, before the rigging tube is finished, make sure that you have at least two pieces of parachute cord inside to use as pull cords for when the time comes to start pulling wire.
3.) Since you are doing this much work, now would be the perfect time to re-locate that battery from the rear, and mount it under your console.
That would move weight forward for better balance, and make it easier to wire the console.
4.) Make your camera part of your 'toolbox', and document everything here.
Start a new thread in the Projects section and we might just make it a sticky. :wink:
Good luck!