Questions about chainsaw mills

Sorry I know this is a truck forum but I figured someone here can help me out.

I have an old cabin that has been passed down through the family and is over 100 years old. Putting modern lumber on it looks like putting LED lights on a classic car, it just looks weird.

Most of the lumber used in construction was milled onsite and I'd like to continue that while doing repairs. I don't have the space to store a large dedicated mill onsite so I've been looking at chainsaw mills and I have a couple of questions.

With a rig like this, how do you accomplish the first cut? Wouldn't any irregularities in the surface the log be duplicated in the plane of the first cut? It seems like I would need two parallel rails on either side of the log to make the first cut flat. Am I missing something here?

 

I have limited experience but you can use something like an extension ladder as a guide or just buy rails when you buy the mill. Some people have used 2x6's. We had rails. Start off installing the rails and getting them leveled up, make your first pass and the second pass is without the rails.

Northern Tool sells a whole kit.

 

@Whitebutler

 

Ian summed it up well. I'd also add, don't rule out the option of renting or hiring out a bandsaw mill to cut up a bunch of boards at one time. If you have to do a lot of milling, chainsaw mills suck.