Like someone said,boring it out doesnt make power,it just cleans up the cylinder walls.You'll get about 6 C.I. by doing it.
There are 3 ways to get 450 HP out of a 400...
1)add 2 more 400's
2)rip it out and put a decent engine in it
3)spend a TON of money
These engines were made to comply with strict emmissions in the late 70's-early 80's.They just arent a realistic base for BIG power.Sure it can be done,but it will cost WAY more than if you built a 429,or even a 302/351W.There is just much more available in the aftermarket for those engines,and they are a much more reliable power maker.It makes me laugh when guys say that a 302 or 351W cant make the power to pull their truck.A stock 400 makes 150HP,a stock 302 in a Mustang makes 225.You're already 75 HP closer to your goal.
A 429 will bolt right in place of an M.Basic upgrades will get it around 400 HP for under $1,000.It would takes thousands to realistically get that out of a 400.Anybody that tells you it won't is full of it.
People dont like to hear that about their engines,but it's the truth.Yes,I have a 400 in my truck.It runs good for what it is,but if I wanted more power I would dump it in a second.