There is a lot of info here
http://www.ford-y-block.com
Dropping a 312 crank into a 292 block requires turning down the mains. This is generally seen as an improvement. More iron in the MB webs, to reduce the cracking you found. Depending on how the block sonic checks, the condition of the crank, and how much you want to spend, you can get 340CID. I've got a 292block/312 crank combo that's 320, and a 312block/312 crank combo that's 340. That one needed to clearance the rods from the cam. John Mummert's web site about list all the combos, and pricing, along with more info. As stated, don't throw anything away.