There are four governors available for the C4 transmission. You can buy one at most transmission repair shops and just tell them you need the one that shifts at the highest rpm.
The govenors can be modified but it takes a lathe with a tool post grinder and a bit of knowledge about what you are doing. You can install a modulator pin that is 1/16" longer than the one that is used between the modulator and the modulator valve. You can shift it manually and shift it whenever you like.