Just wanted to get back to say that, hew! it worked like a charm!
And gosh! kibito87! Was that easy!!! Thanks 1000 kilos for your input and of course kudos to Krayzie for such a heck of an installer.
IMHO, this should explanation should get referenced in a sticky post or something, I spent countless hours looking for a solution and kibito87 is clean and EASY.
Just a pair of notes, for the sake of compleness:
Configmagic did not run on my funky XBox. It went to black and stayed there. Not that it might be important, because I was doing all this precisely because of how crazy it was going before trying anything (like booting up to the dashboard 1 out of every 5 tries; it did boot to X3's emergency mode -purple X; requires no HDD- everytime; do not know why).
X3's own "Config Live" BIOS menu can upload/download EEPROMs and lock/unlock HDD, so I did it with it.
For whatever reason, after flashing my funky XBox with the new one's EEPROM and locking the hard drive, the funky one refused to boot with the X3 disabled any more. I decided still to extract the big HDD and give it to the new XBox, which swallowed it without complaining and booted into MS Dashboard just fine.
Krayzie's installer does allow for >137GB in F: and no G:. It's not the default mod installation, though. Once the new XBox has the big hard drive in and modded, one should FTP the "Extras" folder included in Krayzie's rar to the XBox E:/Apps/ folder (username:password being xbox:xbox), rebooting, launching "Krayzie's Extras" from Applications and installing NKPatcher 06 (just like the read me says, duh!)
So, thanks again!