I would try to keep it as far forward as reasonably possible. The farther to the rear you go, the more likely you are to experience negative effects in the balance of the rifle.
Most aftermarket Pic rails utilize the sling swivel stud location as the mounting location for one of the fasteners of the rail. You should be fine.
Here is a custome rifle with a custom rail installed (LongRifles Inc).
Hope that helps,