First, go over to car craft.com and read the article about the 5.3L Chevy. With just a cam and valve springs, they added 96hp and 30 or so lb ft tq. next, they tried 3 different cyl. heads with the cam and springs, the best heads and cam..... increased hp from 335(with "f"olley 750 and headers) to I believe 460, engine hp, and 396 tq. The 5.3 is a little better per cubic in than some of the other vortecs because it uses essentially the same heads as the 6.0 so it has very good flow #s especially the exhaust.
When talking the 427, part of the reason the 427 is a good motor is simply shorter stroke compared to say the 454. Shorter stroke equates to more overall ignition advance without pre-ignition issues. A very interesting article in an old Hot Rod mag that I recently looked up online titled "the 350 chevy should have built". 2 guys made a 352 from a sm bl 400 bored .30 over(4.155) and a 307/327 crank (3.25"), Ford 300-6cyl. rods (6.209"), custom J&E pistons with the wrist pin loc higher on piston, Alum heads with 58cc comb cham(11.00/1.00 compression)and a stock roller cam, and a quadrajet/ headers. With pump 87 oct unleaded dyno #s were, 412 hp and 435 tq. they said it was all in the geometry of the stroke/ long rod combination. More specifically, that combo the piston was at tdc longer (real time) as comp to longer stroke/shorter rod, and that's why they were able to obtain 36deg total ign adv on pump 87 unl. Look at the 302 DZ, that engine was awsome in large part because it had a 4.00"bore and 3.00"stroke.