what needs to be changed in a 72 340 to be like a 70 340?

The best 340 specs, were for late '71, high comp,forged steel crank, "J" heads with 2.02" valve and electronic ignition.
The worst part of factory engines, is as stated earlier, pistons were heavy, and balancing man that was a crap shoot. 'Cuz they sure weren't balanced when they were built, (and that makes a world of difference). I'd always prefer seeing balanced/blueprinted.
Clearances, depends on application, loose for drag racing/high rev, a touch tighter if you want to cruise...

What is your ultimate goal ?