Wait, do you really need another program to upsample? I'm pretty sure Photoshop upsamples by interpolation if you increase the image size. I just did a quick test with GIMP
, and it did an ok job when I quadrupled the image dimensions. I think perhaps I could improve it a little more with some noise reduction and sharpening.
Anyway at 48" x 72", I get a 2000 x 3008 pixel native print being 42 dpi. A monitor should be have about 2.5 times as many dpi, for comparison.
Therefore, you can get a rough estimate of what the print will look like (without upsampling) by zooming in on a section of the image at 250% on your monitor. Step back until you can't see the pixelation and let the gentleman know he should be aware that very critical observers standing closer than that distance may notice the quality limits, but you'd be happy to sell the print to him as long as he's aware of that. Most people are so caught up in the scale of an image that big that they don't really know how to look for softness up close.
In my test image, it looks like the pixelation becomes difficult to discern from about 5 feet away.
And while we're on the topic, no existing camera is going to give you 300 dpi in a 4' x 6' print. Even a $25,000 Hasselblad H3DII-50 with a 50 MP
sensor is going to print that at barely over 110 dpi. In other words, while a larger source file would be ideal, even professionals deal with the same problem, so don't think your 6 MP
source file is a deal killer.