I have a 408 in my 78 F150. It is nothing more than a 351M block bored to overhaul size, new pistons and a 400 crank.

I might point out that Horsepower is a calculated figure based on torque at a particular RPM. To have a high horsepower number usually requires an engine that has a torque peak at a high RPM. These engines do not lend themselves to high RPM work. What they ARE capable of giving you is TORQUE and LOTS OF IT! To get 400 rleiable horsepower from one of these engines will cost BUCKS.

For a modest investment OTOH, you can easily build LOTS of TORQUE.