Rebuilding a 340

could use original motor. the early 68 -70 340 has a better rod in it. 496 casting. these are stronger over the 645 casting. freshen block with a bore and hone with a deck plate, makes for round and straight bores= more power. install the mopar SST valves in the heads. some mild bowl blending will produce more air flow. I would use a cast or a forged piston. hypers require way too much ring gap. and are too brittle. the comp cams extreme energy cams are the way to go. very aggressive lobe shapes. can still use your original rocker arms. for a starter, use the extreme XE274H. intake manifold, stocker will produce good power or go to a edelbrock dual plane.
to run mid 12,s will require some suspension work. chassis is ET. I would go with the calvert bar/ spring system. it bolts on! you have a four speed trans. install a lighter flywheel. mopar sells a L/W steel wheel or use a aluminum flywheel. clutch type is important. the mcloed B&B/long pressure plate is a very good unit. George at Mcloed can fix you up with correct clutch. forgot, use a moly top ring set, get oil ring drag down to 16#. just food for thought. I work on a 72, 340 duster, stocker, it runs mid 11 secs.