Well 6.9s are not really good engines to make a lot of power with. The biggest factor being the ability to hold the head to the block even with studs. The second being able to flow tons of fuel. It can make some pretty good power at the expense of a big hole in your wallet. If you are going to you would need to drop the compression ratio by shaving the pistons and getting head studs. Then you could run in the 20lbs of boost with the wastegate shut. To get past 30lbs your are going to need to have your pumped worked over and some custom injectors. The big problem though is when you drop the compression to be able to run that much boost then it gets hard to start in cold weather. Wish you well though with your project