For pretty-much ready to fire up cast iron engines we generally use 575 and 685 pound figures for weight. So a big block is only 110 pounds more and aluminum heads drop a quick 42 pounds over cast iron heads. That is 42 pounds of a 52
48% differential on a 2500 pound car. If your girlfriend gained 42 pounds you would know it pretty quick but on a street car so what.
If you use 1 HP per cubic inch just for sake of argument (1 HP per inch is a fair benchmark I think for custom engines and not that far off for a performance crate engine) Depending on money spent a ratio of 2 to 1 is sure possible. I had a $$ Shafiroff 540 in my own car, aluminum, injected and all the goodies, it gets crazy quick. The weight difference between a small block and a big block gets lost in the math of horsepower per pound VERY QUICKLY.