What kind of thickness numbers should I be looking for when I sonic test a 440 block? Motor is going to be bored 55 over.

You'll get a wide spread of opinions depending on the build. I say anything over .200" on both thrust axis after boring is a bonus. A lot of guys have gotten away with thinner numbers (.160") on stock/near stock rebuilds.

Make sure the block has been tanked and as much scale removed as possible in order to get good readings.

I have sonic tested a bunch of 440 blocks, the average seems to be near .160 or thicker before boring on the thrust sides, if it is less than that look for another block if you want to make big power(700+HP) with or without a power adder. If your wanting to make big power by a new race block like the World or Koleno blocks I have seen a bunch of 440 blocks split the main webbing when you start making big power with them

Yes, a new block would be nice , but you read all the problems with world blocks and the expense of the new blocks $3000.00 and up , then add machine shop prices , its hard for the bracket racers to come up with that cost. I wish the block mfg. would look at the cost and find a way to lower the price, look @ chevy & ford cast block and they are around $2000.00, and I know they sell 10 times the amount of mopar blocks, but if the mopar blocks were this price , most of us would stop looking for used blocks and purchase new blocks. Mopar blocks will never reach the sell volume as the other brands when mopar blocks are twice the price. I hate to look for a used block for 750 hp , but hard blok and new caps, and girdle , and about $800.00 in machine shop work I am back at the racetrack.