Transparent Background for T-Shirt Graphic

I’ve got my design ready in a high-resolution PNG, but the white background has to go before it heads to print. The challenge is that the artwork also includes white lettering I want to keep perfectly intact—only the other white areas should turn transparent. No colour tweaks, extra text, or fancy effects; this job is strictly about precision masking. The image is of wild horses with a mountain, sky and clouds, and the grasses at the horses feet. I need all that intact. I just need it to be transparent so it will blend on a tshirt with the text remaining a solid white.

Send me back:
• A clean, print-ready PNG with transparency, while every bit of white text remains solid.
• (Optional but appreciated) the layered PSD

Please keep the resolution exactly as the original and return it as soon as you can—my printer is waiting.