68 340/68 valiant ?

That sounds like a good street combo, the cam is like a street hemi grind and the torque converter is not radical. Unless you plan to do a lot of street light to street light action I would keep the 3.55, it`s enough to get you going but not so much gear you can`t go out of town. Although I agree a 3.91 or 4.10 would make for some great hole shots you will probably appreciate the 3.55 in the long run.