^^2. The 312 blocks aren’t that desirable unless you’re doing a restoration and the casting dates match up. Good cranks are a premium, to some extent rods and pistons.
Along with cracks in the main caps, the 312 blocks have a tendency to have cam bore alignment issues. I bought a nice running, cheap rebuilt 312 mainly for the casting number (56 Mercury). The crank failed the mag check, the cam bore was an issue, and it had been align bored before, so it needed a custom timing chain setup to take the slack out. I was building an F code clone with the pass car supercharger set up, so I couldn’t cut any corners. It all came out fine, but a lot of $ later.
Here’s a post from the yblocksforever site, current, about the rods,,crank, heads, etc. You’ll find more info there on the issues I’ve mentioned.
For top dollar, I’d be prepared to pull the heads and pan so bore and main caps can be checked and take it from there. John Mummert’s site will give you info to check the casting numbers out.
No offense, but all y blocks are 312 Thunderbird motors with the good heads until you start checking the castings.
http://forums.y-blocksforever.com/To....aspx#bm144925
http://www.ford-y-block.com
Bottom of the page, “information center”