Gas Smell/Leak On Fill Up

Howdy - I have a confusing problem regarding my 90 S15 Jimmy. When the tank is over 3/4 full, there is a strong gas smell in the cabin and _something_ leaks from the front right of the truck (like where the actuator for the 4x4 is). I'm pretty sure its fuel and I know its not oil. Anyways, once the tank is around half or less the smell starts to go away as does the leak.. On a possibly related problem, if you fill the tank to the brim the stupid thing appears to overflow. A considerable amount of gas comes from what looks like the top of the tank (not the filler hose) Now I just replaced the fuel lines coming from the tank (it overflowed before that too) and everything seemed to be completely fine (the tank is quite new) Is there something I'm missing that would cause the tank to overflow like this? Any help with either of these problems would be greatly appreciated..

Thanks
Ben