Finally got my 340 motor back from a friends house who had it for too long…we had agreed to a price and he was supposed to be building it… long story short it never happened and I just found out today at the machine shop one of the cylinders had pits and would have to bore it out, problem is it was already bored .30 over. Is it a bad idea to bore it any more? Or is it safe to sleeve just the one cylinder? I’m trying to keep from breaking the bank, but want a reliable motor. I’m not at the drag strip every weekend and it’s not a daily driver but I would like to have a fun reliable car for shows and occasional drag strips. Or does anyone have or know of a reasonable deal on a good block or something rebuilt and can be tested or driven? Thanks just trying to sort this out sorry for the long post. Ready to get my 71 Challenger back on the road!

As with all questions concerning overboring, only an ultrasonic test will confirm the safe overbore size. Sleeving only one cylinder is a common practice. Depending on where in the bore the pits are, you might consider leaving the bore as is and call them "oil reservoirs".

I always heard 340s had thin walls. I've only sonic checked one and it was very thin. Same deal as you have...it was a .030" over 340 w/ rust pits in multiple cylinders and it would not tolerate another overbore. Honestly, some were marginal at .030" over. Not worth slamming a bunch of sleeves in to save it.
In your case, I'd have no problem sleeving that one hole and running it.
With what people want for a good 340 block, sleeving one hole is a much cheaper alternative.

Thanks for the replies, yes I would agree on the ultrasonic test to confirm. I was just trying to see what the general consensus was on sleeving mainly. I was trying to stay away from going .060 over. I did have my friend who is also helping build the motor for me go and look at it too, so Im going to figure out what his thoughts are.

Installing cylinder sleeves can cause distortion on the cylinders next to the one being sleeve unless the machine shop knows how much clearance(very little interference fit, .001 to maybe .004) to use and has the sleeve chill in either dry ice or a good refrigerator freeser before installing it. You will need a decent dial bore gauge to see the before and after on the cylinder next to the sleeved one to see if they are changed after the new sleeve is installed
I've had more than one block sleeved either on the four cylinders sleeved on the one side of the block or both sides depending on what the customer wants to do after explaining the possible issues with using sleeves.
I had one 400 block that was really bad (casting shift, really thin on the driver side cylinders on the cam side of those cylinders, less than .035 in several places after rough boring it for the sleeves ) bad enough we couldn't make sleeves seal properly on the bottom of the cylinders, had to throw that block away
IHTHs

things I know about sleeving a 340.
A sleeve at 4.283 will typically pin hole the bore.
adjacent bore(s) will require boring/honing of at least .003 to correct out of round.

unless the pits are in the very upper 1" of ring travel, and significant in size (width of ring) forget about them.

You can buy .040 over 340 pistons from a few vendors, but they may likely be a custom order as many go from .030 straight to .060.

Video of a 396 block being fully re-sleeved I watched a few days ago. This one was in pretty bad shape when they got it. These two have other videos that are pretty interesting to watch if you like engine machine work in general.

https://youtu.be/G7M1UC5PVZ4?si=FxviJydTJNNXghQD

I ran mine 60 over for 6 years in my race only Dart. Never had a problem until it blew up. For 2 years, I had some used super stock pistons with the domes milled off. Then I bought some new TRW 13:1 slugs. It finally broke a rod near the small end. It quit right at the finish line. Externally, it didn't look like anything was wrong. When I got it on the stand and rolled it over, it sounded like it was full of gravel.I had to pry the pan off of it. There was nothing left inside. I was able to reuse the valve covers and intake.