It should be X1900GT Mac ROM (BTW, I'd like to have it too). 1900/1950XT and XTX are using other memory specifications and Device ID than GT. I presume that OEM Mac ROM would need some adjustments to work with other than GT 256MB version (if would work at all).
Sure, coolers swap will be easier, but if you want to try messing up with ROMs, here is what you can do:
Dump ROMs from GT and XTX. Backup those files. Compare GPU clocks and memory frequencies between them.
First, you should change Device ID in GT ROM to match XTX DID. Use HexEdit or Hex editor.
For X1900GT 256MB DID is 7249 IIRC. For X1950XTX is 7244 - but check both yourself, because I could be wrong.
Device ID in ATI ROM is always in third line, after 02 10 string. It's endian flipped, so if you see 49 72 after 02 10, DID in ROM is 72 49. DID is repeated in line 8, also after string 02 10. Just replace DID in ROM with one you got in XTX (in both lines) and try to flash to see what will happen.
Second thing is memory: amount and frequencies and GPU clock. GT uses lower clocks than XTX, so this isn't most important at this stage. If you get any image at all after flash with only DID changed, it will be time to make other adjustments in ROM.
And remember: always backup your original ROMs. It's crucial thing.