Viewing distance Image

First of all, you need to understand that ppi is not an absolute, it's not a property of the file. The file itself is only pixels.

When you print a file you need to set a physical size. That's when ppi comes into it, not before, and it's all given by the term ppi itself: pixels per inch. You have so and so many pixels. You want them that big. OK, so that comes out at so many image pixels per so many inches. It's just an equation, a formula.

You don't need 300 ppi! That's for book and magazine print at arm's length or less. In fact, there's nothing special about that 300 number in any scenario. It's just a calculated theoretical upper limit, beyond which no improvement is possible using a standard line screen frequency of 150 lines per inch. It just means you can't possibly make out individual pixels in the final print, not even in theory.

Your file would end up at 36 ppi fit vertically with some space on the sides. and quite a bit less, 23 ppi if you want to fit the sides and crop top and bottom.

That's not a lot, but it's what you have. It'll probably work fine if you prepare the file well, reduce noise and clean the edges up with careful sharpening.

You can upsample to a higher pixel count just to make you feel better, but it won't do anything for the image. It might easily end up looking a lot worse because of artifacts in the upsampling process. It will usually look better as it is.