340 vs 383

Just finished welding in the qtr pnls on my 70 Dart and will be turning my attention to the engine. I presently have a tired 340, with a rebuilt 727, hooked to an 8 3/4 rearend. Having access to a 383, I'm at a decision crossroad as which route to take. What entails a 383 conversion, will I gain that much in torque and/or HP? Also, is their a big weight difference, affecting torsion bars, braking, and handling? Any experienced input would help. Tnx: Howard