You should do it separately from or after your lifting. Stretching before lifting will decrease the force you will be able to produce.
As far as supplementing your lifts, you only need to stretch if you can't use proper form or get full range of motion. Additional stretching will not help your lifts (won't necessarily hurt, either). Starting Strength, a program with similar lifts to Stronglifts, only gives three mentions of stretching in the entire book.
So, your stretching routine depends on what you're looking to improve. You only mention two performance metrics in your question: sit reach and splits.
For train those two aspects, do a general full body warm-up (doesn't have to be long... 5 minutes is fine), then do sit reaches, one legged sit reaches, splits, split variations, hamstring, and groin stretches.