340 question

To answer the original question it is the same block. You just need to examine/measure the individual block. Measure if it is overbored and by how much. If rusty inside, be careful, but if assembled, hopefully that's not any issue.

The late '72 and '73 340 cast cranks have special imbalances in both ends, put into the damper and in the torque converter area. (Called 'external balance'.) So not all aftermarket dampers are available with that feature and you have to make sure the extra torque converter weight it there, or change to an imbalanced flexplate like from B&M. So just be aware of that; I just bring it up as any original damper is suspect with the rubber ring probably being rotted out by now so you may be needing a new damper for sure.

Cast cranks are pretty good; we're running somewhere around 375 HP with a 273 cast crank in a 340 (street use only). You could put a 318 crank in there, cast or forged, and using KB243 pistons, would have a good street compression number with the stock heads, and the rotating assembly would be within factory range for balance with the lighter pistons and no balance work, and NOT need the external imbalanced damper and TC/flexplate.