Getting it to crank is nothing, IMO. Easy-peasy. What the relocate really messes with is charging system performance.
A resistance that causes 2V of drop during 100Amps cranking will cause 1V of drop at 50 amps of charging. Starters function just fine at 10 Volts, batteries will not be charged more than 25% at 13Volts, and won't last very long because of chronic undercharging.
The alternators on these trucks regulate the output at the output terminal. Any resistance after the alternator (or in the return ground side) will not be compensated for by the regulator. If the set voltage is 14V, at the output, it will be lower anywhere else in the system regardless of how many or how big the wires are. The greater the resistance, the higher the voltage drop.
There is a terminal on the regulator that can be utilized for remote sensing. The regulator will control output so the set voltage is achieved where ever the other end of the sense wire is connected to.
In my hypothetical 1V drop with a 14V set voltage, connect the sense wire to the battery in the rear of the truck. The regulator will now control output to maintain 14V after the drop. So there will be 15V at the alternator output terminal. Seems good, eh?
Now one more thing. The tuning gurus know more about this than I do, but injectors flow differently depending on voltage. So if your injectors are receiving different voltage than they were tuned with, or the voltage correction tables in the .bin are incorrect, your fueling will be influenced by any changes in system voltage caused by battery relocation and associated changes. Because the injector voltage originates at the rear of the alternator, with 15V instead of the 14V it was tuned with, the injector will deliver more fuel. Again, there are correction tables for voltage, but that doesn't mean they are correct for YOUR injectors.
See? There's a LOT to this isn't there?