So, manual or auto transmission?
It'll be much easier to achieve 250 whp with a turbo setup, and your transmission should be able to handle it. Manual will need a high performance clutch, but I don't think any major rebuilding would be needed.
The main thing to worry about is building the engine. You don't want to have any valve float or parts exiting the engine. Forged internals, new valves and springs are all a good idea.
Believe me, these cars have a habit of growing on their owners. Your feelings will only strengthen over time...