I know that the stock ls1 crank has been proven to hold up to fairly impressive power levels with no issues but is there any way to make it stronger?

What can you have done to your stock crank to make it "hold up" better?

Also, is there a difference between the LS1 and LQ4 crank, if so, which is stronger?

When a crankshaft is overstressed by too much power, what part of it actually fails?

 

If you're running an aftermarket rod and piston, you should have your crank balanced to ensure longevity. They will most likely have to add heavy metal to balance the counterweights with the rod/piston combo.
It can break between the mains or on a rod journal... I've never seen one fail though.

 

A guy I know built a SBC, not an LS1, for his teen age kid to drive. The 2 bolt main, cast crank core for this engine was just a generic 350 out of an old Van or something, but they built it to over 400 hp. He told his son to keep it at or below 6000 rpm. The crank sheared off after the kid beating on it for over a year. The kid's friend said he had been reving to above 7000 rpm for quite some time. I was impressed that a junk motor built in this guy's home shop held up that well, but I think they did balance it. They did everything themselves and had very little money or special parts in the motor.