Best place to add a large ground wire??

I want to add a nice double bus under my dash for adding future amps and custom ballast. I have taken a good look at all the electrical, especially the battery cables. My boat is an 02 SANTE. It seems that the only power available under the dash is off the black box, which has the pos and neg terminals and several light gauge wires on them. I assume that it takes it's power from the 10 ga wire on the dual battery switches. I don't want to tax this system and prefer to set up my own buses with large gauges from the battery switch and a ground from the engine. I figure wire gauges for 100 amps will be fine (2 gauge) and I doubt it will see that much but better to oversize and do it once.

I have located what I think are two grounding studs on either side of the engine and one on the aluminum engine mounts. The one I am considering is located beneath FCC, it has the rear battery ground on it as well as several other smaller gauges. The front battery ground runs through the bow and into the hole below the dash and on to the aluminum frame that holds the engine where it is bolted down.

I am also considering just splicing the front battery ground and attaching it to a new bus bar , this would be easiest but I don't know if it might over-amp that wire, it's 2/0 but runs the length of the boat. The shortest route other than the splice would be to attach the new ground on the front battery, but that again taxes the front battery ground.

So I think I should either ground to the engine ground under the FCC or ground to the stud on the aluminum motor frame. Any thoughts on this?? I know this is lengthy but I don't want to cause any damage to the existing system and this is the safest way I can think to do it. Is the aluminum frame as good a ground as the engine ground??

I will take my pos power right from the battery switch studs.