if you are going twelve volt then keeping the 27si alt is not needed anymore. you only need it for the 24 volt alt. but since you need more power just change the drivers side alt to a new model like:
We have developed a high output alternator in a case that is smaller than the early GM 10SI case. The mounting on this alternator is exactly the same as the Delco 10SI, 12SI, and CS-130 cases or we have a 1 pivot ear that is common to AMC and marine use. It has the following benefits...
www.qualitypowerauto.com
This will solve your power problem. yes the banks kit will do wonders for the truck. I thought however the banks would still fit with both alts but I am probably wrong.
You could go extreme and do what I did to my 1009 but it takes some time and fab skills to make the elbow for the turbo. There is room for the exhaust to still go under the truck but I did not take that route.
The banks kit does make running the cross over a little difficult without hitting the axle. I am using the banks manifold but I changed the crossmember and motor mounts to the ORD parts and the exhaust crossover runs next to the oil pan above the cross member. The parts are not that much to purchase and it allows the cross over to not interfere with the axle or drive shafts.
If you could make the 24 volt system work. I would stay with it. Most of the new LED lights can use the 24volt input and then you can still use some of the military lights also.
I am still putting my roof rack together (work keeps getting in the way) but with my military 24volt IR lights and my $200 night vision video camera I can drive in the dark. (only off road, not used on the roads). Since my truck is not finished, I have tested it with my civy truck and you can see really good. Great for varmint hunting.