There are three things that control the rpm shift point on a C-6. I would not be surprised if a 4200 rpm shift point is the result of a perfectly working "stock" C-6 trans. Want a higher shift point? You may have to tweak your "shift kit". I'm guessing it's your "modulator" thats the problem (assuming your shift kit is installed and working correctly).
Valve body: Controls auto or manual shifts depending on whether it's "stock" or "modified" for performance, rebuild/modify kits are available. This is the most common "modification" on a trans for perfromance. The
VB is what your gear shift acts on to select what "gear" you want.
Modualtor: Adjustable, usually controlled via the throttle linkage that goes down to the trans. Should be fully "engaged" at or near full throttle. If NOT, "early shifts" WILL occur.
Governor: At a certain rpm "over rules" the modulator and forces a "shift" anyway. It's action can be "over ruled" by the valve body. Generally not easy to modify like the
VB is.
The modulator MUST be hooked up or the trans shift points will be a nightmare. Raising the shift points increases the internal hydraulic pressures that act on the clutch packs and the "band" for 2nd gear. A "race" trans will have more aggressive longer wearing clutchs, the "band" and the planetary gear set is also "beefed up".
A stock C-6 with a 400 hp engine in a light weight Cobra SHOULD hold up OK. More than 400 hp COULD result in early failure.
Ernie