The question of building verses buying, I doubt you can build an engine like you want for the price of crate type engine. You will also need to know what parts to buy, and thats a bigger factor then the actual building of it. Most guys can build engines all right, it's choosing the right valve train, induction system, rotating assembly, and drivetrain that gets you in trouble. The transmission and rear axle all have to be apart of the engine selection. Once you nail that down, now you need to stop the truck, cool it, and keep it fueled.
I would really want to study the dyno sheet from the manufacturer before laying down a dime. If the 500 hp is way up in the rpm range and the low rpm is flat, then you would have a very poor running truck till you spun the engine high enough to be in the power band. 97% of all driving is off idle and cruise, the other 3% is burn outs and a few high speed runs.
As a point of reference, my dad's '71 Pontiac wagon is a street driven race car. Its street legal but set up for drag racing. Engine is a 478 ci Pontiac making 555 hp at the flywheel. The wagon has 52% of its weight over the rear tires, thats 2050 lbs. With a decent sized street radial, it will leave black marks through first and in to second gear from a slow roll. With drag radials, the car has run as fast as 11.32 @ 118 mph in the quarter mile. To do that it has a bullet proof T400 transmission, and 12 bolt Chevrolet rear axle with all aftermarket racing internals. It took lots of money to put that 500 + hp to the pavement and a lot of trial and error.
