It’s expensive to correctly rebuild any engine and even more so for an engine from the leaded fuel era. And adding a core 312 into the mix makes the value go up even more so. While I’m not a 312 block fan, a 312 crankshaft core alone can bring up to $400. A rebuildable 312 core engine that’s complete can be worth up to $1000 or more so you see where this is heading. Knowing who rebuilt the engine and what parts were used would help in determining a suitable price to offer. A simple re-ring and bearing job is obviously not worth as much as a more thorough rebuild where the block was bored, crankshaft was ground undersize, the camshaft and lifters were replaced, etc.

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If you do end up getting this engine/transmission combo, then realize there is likely going to be no warranty if this engine is not immediately installed in a vehicle and used. In fact, it might help you in your offer to make the offer with the understanding that you expect no warranty and will not hold the seller responsible for anything you don’t catch at the time of the sale. This drives from the fact that the current owner of the engine is actually not wanting to sell the engine and it is you that is actually initiating the sale.