I have the same exact problem, I'll have to inspect this sending unit seal/gasket.

Did you see any fuel at the source of the leak ? I have gas odor after my 311 is closed up ... the sending unit access is at the rear of the tank in the engine room ... I havn't noticed any fuel around the sending unit but there is definitally fumes coming from somewhere .....thanks .....m

Sending unit gasket seal was bad on my Fountain as well...topped the sucker off right after I bought it and had it sitting on my lift with the bow high in the air for an hour or so while I was adjusting the lift...ended up with a couple gallons of gas in the bilge...NOT something I'd recommend! Fixed the seal, fixed the leak. Now I smell gas ONLY when it's coming out of the vents because it's that full.