OK, I guess a lot comes down to what are you basing your needs of 400HP and 450 torque on? Are you comparing it to another truck you have/had, or just numbers you grabbed out of the air? How much money are you planning to put into a motor? Also while it might be better to start with a big block, you also need to add not only the engine, and trans, but everything else needed to swap it in. If your looking at a fresh build I would look to starting with a 5.9 mag, and building that. Lots of talk on how they respond to stroking them also. A good build focused on lower/mid range power, and drivability, and then run it. I have found when I shoot for a number on a build, I normally loose focus of what I really want out of it. I guess the saying "you dont drive a dyno" fits here.. Low cost do some basic mods to the LA360 in it, or higher cost build a 360mag and sell the LA to someone with a older truck, and let the numbers fall where they will.