A popular cam upgrade would be the Comp XE268 on a stock type rebuild.
The 340's were not torque monsters so a good performance upgrade from the stock setup is a 3200-3800rpm converter if this is an auto car.
I would vote on the 3:55/3:91 gearing depending on the rear tire size.

1970 YO7 A66 [Canadian Export] F8 Challenger
340 (Currently in shop for stroker assy.)