Iphone 3gs custom update

I have a white iphone 3gs 16gb running 5.1 firmware (9b176). I have downloaded 6.1 firmware as i want to preserve the capability to jailbreak phone.

I have used itunes to update phone, have also hit restore but keep getting message, this phone cannot be restore as device is not eligible for requested build.

Have also tried 6.1.2, same message...

Am I doing something, I would of thought this would be relatively straightforward, with ios7 being released means there will probably never be a jailbreak for 6.1.3 which is as far as the 3gs can go...

Have found this method but tinyumbrella the box is already unchecked, and my host file already has the # present.

Fix “This device isn’t eligible for the requested build” Error in iTunes When Updating iOS

Anyone got any ideas....

Ive also went to the private/etc folder and deleted the gs apple.com line but problem still persists..???