Personally I would recommend a ERL re sleeved short block for 427ci or over. People have had issues with the 4.1 inch stroke on a stock LS3 block. With the ERL short block, after market LS7 heads, matching cam and Fast 102 intake, I think 750 crank horsepower is very doable.
BLK/BLK 1SS/RS Ordered 11-01-2009 Took delivery 12-22-2009. Heads/cam/converter/bolt ons. SOLD Feb 2015 to fund 6th gen LT1 SS with 8L90E.