When you open an illustrator file in Photoshop, you can set the DPI. That is not a problem. If you then upres the file. THAT is a problem because after the file has been imported or placed, it becomes a raster file. This is a similar process to using the type tool in Photoshop. Type will scale just fine until you 'rasterize type >>'
To make sure you can print at size at 300 dpi, you might consider doing the following.
1. Create a photoshop document at size (26" by 8.25" or whatever) at 300 DPI or higher.
2. Choose "file >> place" then place & position your Illustrator file wherever you want, and scale to fit. When placed the first time, it will absorb your 300 DPI at the correct size, resulting in a reasonable printing resolution. Once it has been placed, though, it is now a raster (not vector) file and can no longer be scaled up without reducing quality.