Some 340 Questions
Fstforward -
Everybody so far has focused on the engine and rear end gearing. My concern is the rear end itself (the cars, not yours - you're just waiting for your Hooter girl spanking).
You're gonna need a 8.75 behind a stout 340/4-speed combo. Nothing's been said so far about what is in the car so I assume it's the original rear end that was behind the 273/4-speed combo.
Something else to consider.