I've never read through the Roscommon conversion, so maybe this was already covered, but just in case.
If you change to 12v, you will need all new cables. You are trying to do the SAME amount of work (same power required) with LESS voltage, which means you need more CURRENT.
Power = Voltage * Current. Drop the voltage by half and you must double the current to get the same power.
More current needs bigger cables.
And for those who are thinking that a 12v starter means you need LESS power, nope. You are still trying to do the same amount of work - turning over a 6.2 diesel engine fast enough to get it to start. The amount of work that needs to be done has not changed, therefore the power required to do that work has not changed.
We see quite a few people on this forum who complain that their trucks don't start well after going 12v. It's not because there's anything wrong with 12v, but I suspect their problem is they didn't get bigger cables.