If you are going to do the job, make sure you buy marine tinned wire, a good crimp tool, quality crimp connectors ( the heat shrink kind with sealant) and a heat gun.
In some cases, it may be possible to disconnect an existing wire bundle, pull it ( tie a drag line to it for later reinstallation ) and use it in the shop as the template. Whether or not this works depends on many things like spots where the cable may be secured but you can't get to the clamp, stiff cables that no longer flex and perhaps connectors that are too big to pass.In that case, you may be forced to cut the connectors off before pulling and reconnect after the new cables are installed.
Take really good pics and notes. It may be next to impossible to locate OEM engine harness connectors so if you are replacing motor harnesses or gauge engine specific gauge harnesses, make sure you keep the connectors and sufficient lengths of tail wires to reconnect. If you have to splice a harness to a connector it is often best to stagger the splices by making the tails different lengths so that all the splices don't occur in the same place and make a fat bundle of splices . Bundles, if present can be heat shrunk wrapped using wide tubing for a nice look
As you probably already know, do not be tempted to solder the connections. Good quality crimps are the way to go. As mentioned, follow the guidelines for bundling and securing the cable runs.
One more thing to consider is that if you are relocating the fuse block, you should see if that adds overall length to the battery accessory feeds and grounds and if so, will the current wire gauge be correct for the new cable length. Larger cable sizes might be advisable.
Sounds like a 'fun' off season project. Good luck