360=500hp

There are plenty of engines making 600hp with some factory parts. The block is all I'd use. The reason is easy. It's an air pump. There's only one person who I beleive has the ability to flow the air required with factory head casting. HP is a function of torque, which is applied twisting force over time. That means you can make more power by increasing the force, or increasing the frequency of the power pulses. Increasing the force is compression, cylinder pressures, stroke changes, bore changes, and all good machining practices(more stable parts allow less loss from inaccuracy). Increasing frequency is rpms. More rpm=more air flow needed. The book "The Auto Math Handbook" has some great sections about piston speed, hp and torque, and efficiency, and is written for the hobbyist, as opposed to the engineer.
As far as possible, take a look at many of the faster Super Stock Eliminator (not Hemis..the other ones..lol) cars, and you'll see it is possible. A friend's SS/GT mustang runs a 351+.030 Cleveland. Factory head castings, factory block, factory crank, factory compression. (not sure if the factory carb is used, but it's no matter) Rods, pistons, valve train, and intake tract are all aftermarket. It made 685+ on the dyno a few years ago. But, before it's misread...He has over 300 hours in the heads alone. (he now owns a flow bench in his garage at home..) The ports were completely reworked, the valve train is the result of at least 10 cams I know about, and the peak torque is 7K rpm, and peak hp is 7800. Street car? Nope. Budget? dont ask, he gets real testy about that.
The thing is, you dont need 600 hp to run fast on the street. Look at the ETs of the posters here. less than 500hp running low 11s. The more streetable, the less power in most cases. I dont agree with the "you have to build a 600hp 440 to keep up with a 500hp 360". I know of several 530tq 450hp 440s that routinely walk all over 500hp small blocks. On the street, Torque moves you; HP is a bragging number. I do think the way to build a killer A body is a small block. I'd build the biggest you can afford, because bigger is only slightly more or the same $$, and gets that streetability back. The lowest budget 416 I can build will cost less to build and run than a 360 cid that needs to make the same power. The coolest thing is, like they've said..No new trans case, special headers, special mount kits, and equal handling/braking to a 318 car.