power gain using lightened crank and aluminum rods

As was stated, they won't be worth hp and won't show up on the dyno. But they will be worth ET as the engine will accelerate quicker.

As for Rods... I'd look for a billet steel rod from Crower, Oliver, Carrillo.... of course that has to fit your budget. Aluminum rods suck unless they serve a purpose such as a cushion in a nitrous engine, or very light components needed in an n/a engine.....

As for the lightened crank.... I'd call the crank manufacturer and have a chit chat wtih them.... I know a SBC ultra light Bryant crank, they'll tell ya it'll take 1500 horsepower or more.... The big block just by nature is going to have more mass to it, and more structure so I recommend talking to Bryant about it... give em a call.