I would absolutely stay with the 400. It's already in there, no modifications to do, and no headaches. Trade the 350 in on a quality rebuild on the 400 and be done with it. The 350 is a great unit, and yes, it does take a little less HP to turn it in high gear than a 400, but in a street car, you'll never feel the difference, and the fuel mileage won't be that much different, either. It don't make much sense to go backwards with this swap! You can use a 350 torque converter in the 400, if you want to save a little weight (10/12 lbs). If the choice is 350 or 400....stick with the 400. my2c