I have one on my truck. You can use it either way. I like that it has an O-ring instead of a gasket. What year truck do you have?
It's the same design tube that is in most fuel sending units, just larger diameter. You shouldn't have a problem with it falling off.
It's a 99, which way do you use it? I'm somewhat worried of that plastic line falling off in the tank
I used the suction tube and the bulk head fitting. I was not able to place it under my sending unit bacause of the was my basket is made. I put a kink in the tube.
Works fine. I have ran it lower than an 1/8 of a tank with no fuel pressure issues.