RE: First experience with Bubble Jett tanks
The carb is low on a side mounted engine so you could get siphoning regardless of the tank.
If you do a good job of drawing all the air out of the bag, then just fill it with just enough fuel to avoid any pressure build up, you should be good to go. With a 3 line set up, I don't bother with sucking the air out, I just fill the tank until fuel runs out the overflow line, then plug it off.