Doesn't look bad . Ask them if you can get copy of sonic test results. If they say we don't do that. Ask them how they know how thick the walls are. If available pay extra for sonic testing.
I built what is probably the largest Factory Small Block chevy anyone has done with factory cylinder walls and factory block at a big 440 cu in. I did it once. Probably will never do it again. At that time no one was making any special stroker rods like they are now.
LIke I said, it can be done. I do not suggest o put much nitrous on it if you do though.
Many years ago a guy built a 452 cu in SB Chevy using a factory 327 block. That one was something else and very expensive to build. lol.
The core shift tolerance from the factory on the 400 blocks was beyong belief, it was so bad. One block may have thick walls all around and could bore to a 4.185 the next ten might not be able to make a 4.165 bore. and some wouldn't even make a 4.155 bore. Many people try to go by casting numbers to tell which ones are good and which ones aren't. Maybe that makes a difference and maybe it doesn't. I find good 4 bolt and 2 bolt blocks and I find bad blocks in both.
Ed