The forward movement of the axle with a flip depends on a LOT of factors. The biggest is the arch in the springs and I think the 2nd biggest is just how it's measured. With a stock spring, the flip moves the axle about 1/4" which is basically imperceptible without a tape measure. With more arch that can be a bigger number since the spring isn't flat.
Part of the problem is that the tire starts off closer to the front of the wheel well on these trucks (early and later model) so we really need to move it back to start with.
With the early trucks, you typically have to run a spring with more arch than a '73-up so the axle will normally move forward more. There are some games you can play to fix it though, like the Zero rates mentioned earlier and another one is flipping the springs around so that the long part is to the front. Not all lift springs have an offset pin but those that do make it easy to move the axle back a little.
Also keep in mind that if there is some arch in the springs, the axle will move back when it compresses so if it's sits at the front of the opening a bit, it could move to the right place in bump travel.
Hope this helps