I had this block hot tanked and magnafluxed and they found these cracks in lifter valley. Is this block savable?? What causes these? Heat from exhaust crossover? Or?

Looks like it froze at one time.......likely junk

From what I hear it is from water freezing in the block and expanding causing those cracks. Chevy's are known for cracking in the lifter galley area like that.
So my opinion is it's from not being properly winterized.
I would NOT use it since it isn't hard to find big blocks and I don't think the cost to weld it would be cheaper then a different block.
Just my 2 cents.
David

get a new block. no point in spending money on something that is already damaged.

That happened to the original 383 block in my 69 Dart GTS. I had Indy Cylinder Head stitch weld it and everything was fine afterwards. It happened in 1979 when I put the car away for the winter. I put pure antifreeze in it because I was a dumb kid and figured if 50-50 mix was good then 100% would be better...

Yep, it froze. They'll crack like that on the outside of the block b/t the core plugs too.
It could be stitch pinned and saved, but not worth it in my opinion...unless it was the matching numbers block or something.

I used stop leak on a old Imperial that was like
that.. it lasted till I sold it.. I had to change
the oil twice then it was fine... I wouldnt race
a block like that

Throw it is the dumpster and get another block.

Definetily a freeze crack, how is the rest of the block? Usually when you see one there are others I had a 426 street wedge block that was crack on both sides, insides and out as well as having a casting flaw on the bottom of one cylinder that had been leaking into the crankcase The customer wanted it fixed so he could use it in his 1965 Satelite 426 W car I would check with your local machine shops and see what they will charge you to stitch plug repair it The shop I use was so much a inch, it is not a welding process. They drill the crack and insert a twist off plug and then drill out half of that plug and insert another twist and so on until they go past the entire length of the crack It flat works well, no leaks and no problems

Scrap it. Why bother repairing it. It may last, it might not. Why take the chance with $$$ & time involved.

Just use Hard Block and fill it up until it just covers the crack. Then run it.

I tried that on a old top fuel Mopar cast iron block, most block filler are porous and lets the water flow right through it If Hard Block is the plastic epoxy filler it may not let water through, maybe it will
I ended buying some block sealer from Goodson that you had to put in the block under pressure for 10 to 20 minutes and then empty it, it was resuable and have the block or heads cook in a oven for a set time at a high temps, under 400 F if my memory is halfway near correct That was twenty years or more ago that block sealer flat worked, that block leaked like a siv before, not so after that sealer was done

I'd be nervous not knowing what else moved when the crack happened - like from the jacket to the outside of a cylinder wall.

I tend to make decisions that leave me comfortable!

I hear you on that, I've had more than one block stitch repaired with no problems so far, same thing on a bunch of 906 heads I had a industrial 413 block machined to 426 standard bore for my old M.W. NHRA stocker, it had a split cylinder(#3) in it and a matching external crack by the motor mount boses on the driver side that the machine shop didn't find when they, supposidlily, hot tanked and magged that block
If you think about the pressures needed to expand the block or heads far enough to crack them how much pressure is there on the non crack surface opposite of the cracks