I'm pretty sure you did'nt mean lower than the tank itself. You probably meant that it's lower than the fuel draw. If that is correct, 2-3mm will be no problem at all.

I don't know that airplane, but if it's possible, run a long fuel line above the level of the top of the tank to avoid any siphon. It should not be a problem as 2-3mm is not enough fall to create a very powerful draw. BUT, a break in the siphon would help if you are going to leave it fueled for a while before you start it.

Most glow engines run VERY well inverted. In fact, all of my glow engines, except one, (and one of my gas engines), are inverted and I have had no issues at all. The only thing I hate about it is connecting the igniter...and I LOATH remote glow because they break too easily and generally suck.

It will run FINE. Mount it and go fly....

HAVE FUN!!!