David - I'm pretty sure the factory tach in my car is also off a little; I have a 6k pill in the shift light, and the tach reads just over 6200 when the light comes on. I usually try to shift 3-4 right at 6k, but will sometimes hold 4th a little longer before going into 5th. Even with the lower 4.36 gears 5th gear just doesn't have a lot of giddy up in our cars, so I will let it wind up a little more in 4th.

A lot of it depends on the track of course; my engine (also fully built) will turn 6500 all day without breaking anything, but it's not making much power up there - the power curve starts to dive pretty sharply at around 5700-5800. Conversely, power comes on pretty strong in the mid-4k range, so I don't think you're hurting yourself by shifting when you are. Of course, I have no test data to support anything - other than the dyno charts - so I might be totally off base.