PI cam/intake shift points

4800 is really low, even with NPI heads. Stock PI cams on PI heads peak at 5250, i'd think stage 1 cams would at least push it there.

The ideal shift is actually different between every gear change. For example, the jump from 1st to 2nd is pretty big compared to the jump from 3rd to 4th, so you'd want to run 1st out further. This can be calculated by getting your car dynoed and then graphing rear wheel torque vs wheel speed for every gear and looking at the point at which the lines cross. The ideal shifts for a stock NPI would look something like this:
1-2 5500
2-3 5250
3-4 5000
4-5 5300 (because of the much deeper OD gear)
I generally shift at 5k because my syncros aren't happy above that. It occasionally tends to grind on higher rpm power shifts.
Nope, this post doesn't answer your question about where to shift at all. But you could probably shift these rpms up by 200 and have a good idea. You really just have to get to the track or dyno to figure it out for your specific car.

With PI cams and intake on NPI heads, I did my best times at the track by shifting at ~5500rpm from 1st to 2nd, then ~5300rpm 2-3, 3-4 on a 5speed.

When I did this swap a long time ago in my 96 TBird, Modular Depot had the 1-2 shift @ 6500 RPM, the 2-3 @ 6200 RPM. Worked really good for me, 14.1 @ 95 MPH in a car that is a good 600 - 800 pounds heavier than a Mustang GT. My mods at the time were:

Removed Air Silencer, SVO 70mm TB, P&P stock upper intake, PI Intake, PI cams, Steeda UD Pullies, Mustang GT TC, Custom cat back exhaust, 3.73 TL Differential on hoosier 26" Drag radials.

Wasn't a hell of a lot done to it.

I don't recommend WOT shifts into OD or downshifts from OD to 2nd. <-Not a typo...