^^ buddy, are these worth the trouble? I remember reading your post in t-bhp about this, but there I think you were complaining about brightness being low IIRC.
BTW I was looking at the internals of an old car charger I had lying around. It has some SMPS chip inside (forgot the name) which gives a stable output voltage of 4.5V (regardless of input battery voltage), so maybe u can rip one out and use in your circuit to maintain constant brightness.
If u dont have a spare one, u can buy a car mobile charger (maybe 150 bucks or so) to get a stable voltage.
Also, if u r able to source the bright white LEDs from somewhere pls let me also know.