Will ej25D heads fit onto a single overhead block? I'm assuming since these engines were so mass produced it would work but has anyone tried this? The block in my car got crankwalk and I would rather have the engine I have spare than spend 400$ on a block.

Yeah they'll fit. Are you using a 2.2 or 2.5 block?

2.5

The better question is what year legacy is the engine going to be put in.

Use head gaskets to match the heads.

I have the 99 2.5 GT and the engines coming out of a 98outback