450 FWHP + green dot C4 ?

I just purchased a Roush 342 R engine for my 66 fastback They are advertised as 450hp/420 ft lb torque. I am trying to decide on a gearbox for this motor. My car currently has the factory A code 225hp and Green Dot C4, which has been rebuilt, but has an intermittent slippage issue in low gear which nobody has been able to resolve so far.

Given I fix the slippage issue, will my existing C4 be ok for this much horsepower/torque, or do I need to start with a later model C4 ?

Furthermore the gearbox kicks down into 2nd gear when I put my foot down through sweeping corners at speed, thus ruining my perfectly selected cornering line through the apex Can the C4 be manual valve bodied to stop this ?