One of the main reasons they remove RB's, 2JZ's etc is because LS1's are a lot lighter. Alloy block compared to a iron block.
Like the DLS R34 drift car, took out the RB25 Neo, chucked in the LS1, saved 50kg!!! Slapped some twin turbo's on it and you got a hell of a power to weight ratio over the stock engine setup.