If you look at the bottom of this
Strange Axle ordering sheet you can see how many different end styles there are. If your rear was ordered with GM 10/12 bolt ends the LS1 brakes can bolt right on as the pattern hasn't changed. Give you something to work off for measurements too to figure out what you have already.
Provided the ends are all GM style stuff still, you could also have to worry about how far the axles stick out of the end of the tube. Just something else to think about..