Based on your suggested SL500 base for an upgrade:

I would suggest a 300SL-24. Put an updated skin on it from a late model SL. Turbocharge the motor. You can also find them with manual transmissions already mated to the engine.

3.0 liter, 24 valve inline six.

The mosselman TT quotes an output of roughly 335 HP at 0.4 bar of boost. More than the C32, with as much, if not more torque.