Or RockAuto Auto Parts
I'll second the motion to find a truck spec'd to what you want. You can't just replace an engine these days, let alone with a larger block without doing other work.
You'll be best to fine tune the front suspension for the added weight, and for the greater torque capacity you'll need a transmission as the 4.6L won't be able to handle the torque output. I'm not even sure if the bellhousing will match up. You'll also need the 5.4 wiring harness and PCM to hopefully get everything running somewhat smooth.
There's a lot involved especially now that Ford has been using the electronic throttle and a few other goodies. It's more money for Ford in the end and your best bet is getting a truck the way you want it.
What reason do you wish to upgrade in engine size? If you need a boost in HP there are some bolt on mods and programmers that you can easily add another 20-30hp for about $2K or less. Just make sure you keep up with transmission services on the severe service schedule to get maximum life out of the trans!