The portable chainsaw mills do work ok, though you get a lot of waste due to the very large kerf. I wouldn't hesitate to use one to get the log down to a size I can cut on one of my bandsaws. The main difference between the chainsaw mill and a regular chainsaw is the ripping chain rather than a crosscutting chain. The chainsaw mills cost from $50 to $150 for the bar attatchments on up to more than $1000 for a complete setup. Bandsaw mills are a better choice, but the price usually puts them out of reach for most people...$4000 on up to more than $30,000. I use a 20" bandsaw to do my milling, or resawing. If you can locate someone with one of the portable bandsaw mills you could offer to buy the blade if it hits a nail. These blades are relatively cheap...$20 to $70, or so...unless it's one of the big expensive mommas. A ripping chain for a chainsaw is about $15 to $25. I hope you get everything done ok. Black walnut is good wood. I usually cut mesquite.