Are all 340 blocks the same?

Wow! Indeed it does! I like what your doing. IF I could find a thick walled 340 for this project, that is what I would be looking for. Bigger bores have a direct impact of HP. Also, the bigger the bore the better the heads like it.

The bearing speed is a low issue for most purposes. You could also modify the bearing seat for a more narrow bearing shell. This is out lines in the MoPars engines book. A book with many secrets.

The 340 block is exactly the same as a 360 block except the bore size and crank journal size. While the crank journal is larger, it doesn't make it any weaker. As mentioned before, above, where are the stress loads?
If less metal is weaker, then here is a bit of food for thought. There's less metal around the piston in the block due to a larger 340 piston. But yet it holds up great! Torque is the killer of parts far more than horse power.

N.C. is a state with many talented engine builders. If you have doubts for any reason, here at FABO there are several talented engine builders who can help you out. Mike Listen @ MRL (screen name MRL) Brain Halflinger (spelling? Sorry Brian) screen name OU812, (Cracked back can tell you about him) are a pair of fellas here.
"Moper" does a lot of engine work as well. He is a sharp (and a goodfella.)

At 13-1, are you going to run E-85?