This is what worked for me. What follows is an excerpt from my email discussion with David Pumphery at Spartantrailer.com . Thanks David, for getting me going on putting this infomation together!
I have replaced all of the panels below the windows, most of the ceiling and a couple of wall panels. I use 1/8 birch plywood, which is the material used in the 1955 Spartan. After some trial and error, I sanded the shine off all of the old wood and restained it. Then I matched the "re-stained" color to put on the new panels. I first sanded the new panels with 180 sandpaper so they would suck up a bit more stain and stained with 2 to 3 coats depending on the area of the trailer. Due to sun, smoke, age of varnish etc, some areas had darkened more than others. Frequently, I would have to use my stain matching method to mix up a batch of stain that matched the adjacent panels. I would also came back over a week or so to put some more stain on spots that didn't blend well. Not really as much work as it sounds like, once you get your stain mixing down.
The picture shows my method. First, I took a scrap strip of the new birch and the old to my local paint store and had them do the hard work. They matched closely on the third try using 1 part Early American(EA), 3 parts Colonial Maple (CM)and 3 Parts Golden Oak(GO). I took this board and saw how different parts of the trailer matched and dropped the Early American a bit and added some Mahogany. These are all standard Minwax stains that most stores carry. When experimenting, start with a table spoon to measure and scrap wood and always write down the recipe for each try/mixture. It is kind of fun, but messy!
The Alkyd Spar Varnish is just be a personal preference, it takes
longer to dry, but I like the look better.