It may bump the rpm limiter if you just turn off O/D. If it does, then increasing the rpm limit another 200-300 rpm should prevent the 4-5 shift. However, you may want to reduce it to the stock 6250 rpm limit for daily driving.
You can definitely safely leave the O/D off. In fact most people with O/D autos do leave it off when they're drag racing to prevent this from happening. At what rpm are you shifting into 5th exactly? I don't see why you would run out of RPM in 4th causing the additional shift into 5th before the 1/4 mile marker, with just a few bolt-on mods.
Also, you don't want to extend the rev limiter just to finish in 4th. There's other ways like tire size, and gears that are more effective and healthier for you engine. Either way, I don't think you need to do any of these, just turn off O/D and you'll be fine!
How high can I rev my 4.6? At the track my auto is shifting into 5th gear about 50 to 75 feet before the finish line. Car is 4.6, K&N, BAMA race tune, Nitto 255/50/17 Extremes, 3.31 gears. Can I safely leave O/D off? Or should I have tune set to higher RPM? shifting just above 6000 at present. Car runs 13.7 @ 101
2.08 to 2.1 60 foot times.
Thanks guys for the input. I am going to the track within the next 2 weeks so will run with O/D off. Still learning the idiosyncrasy of the Mustang. just an old guy having some fun.