Neck too thick

Mark here is my suggestion.

Harbor Freight or any tool shop will have what they call a contour gauge.
https://www.lowes.com/pd/General-To..._clickID=fb80ca87-356f-4b78-87ee-795cf9a9e5a5

This tool will allow you to actually see the profile on the neck you like so you can replicate it on the fatter neck. I would take the contour at the third fret, the 12th and where the neck and body meet. You can then work your way from the body to the neck or from the nut down to get the contour exactly like the other guitar.

I would use a rasp to take the heavy wood off, and then use a sharp file to get it close to shape and then sandpaper to get the final shape.

Once finished it is up to you as to how you want to put the final finish on. You can stain it, use shellac, you can true oil it or even leave it unfinished and just play it as is.

 

A contour gauge is great for trying to match an existing neck, so I +1 that!

As far as tools go, you have a lot to choose from. Plain old sandpaper will work (slowly), but so will rasps, saw files, planemaker's floats, card scraper and spokeshaves. The only real advice I have is: save the sandpaper for the end, do the heavy work with something like a rasp or spokeshave. If using a rasp or Japanese saw file, make sure to stop well short of your desired shape, because those tools leave deep gouges that you'll have to sand smooth.

I'm not the biggest fan of Nicholson files these days because they're not nearly as well-made as they used to be when they were manufactured in the US. But, they'll get the job done (especially on wood), so a simple solution would be a "4-in-Hand" file. These are available at most hardware stores for under $10, and they've got two file and two rasp surfaces all in one (I think they're meant for horseshoeing). You can use the rasp part to hog off most of the wood, then switch to the file portion to smooth it out a little before you finish up with sandpaper. Woodworking supply places also carry files specifically for woodworking (look in the carving section), but you can also use regular metal files (mill file, half-round file, etc); note that you'll probably want to stick with bastard (coarse) cut, since second (medium) and smooth cuts (fine) are probably going to be a bit too fine and take forever to get anything done.

Lots of options out there, it just depends on what you feel like doing (and what you're comfortable with), and how much you're willing to spend. The 4-In-Hand and some sandpaper will probably be the least expensive combo that'll still make short work of it. As someone who has reshaped a neck, I found that it works best to just leave the neck attached so you can keep holding it and playing it to see what your progress is like. You may get to a point that feels really comfortable even though the shape doesn't match the other guitar, so check your progress often in playing position. Since it's an acoustic the neck will stay attached (obviously), but I'd cover the soundhole with paper and masking tape just to keep dust and wood chips out of the soundhole. You'll still need to vacuum it off when you're done, but at least you won't have little bits of stuff rattling around inside it.