1996 850 gas smell after engine stops running and/or a full tank fill-up

Our 96 850 has about 87K miles and has gave us a strong gas smell after the car is driven into garage, parked, and stopped in last month. We
have tried to found out where the gas smell came from w/o too much luck, though it is pointed to left front of the car, and right rear of the cat (fuel filler area). But, we could not spot a sing of fuel leak at all.
The gas smell is extremely bad with a full tank fill-up. The garage smells like gas every where. I saw small amount of fuel leak around the clamp of fuel tank and fuel filler neck. I removed the clamp and cleaned out the dirt, did it second time to clean the fuel neck and tank contact surface for any possible dirt. (I had to loose the fuel neck at fuel filler door and had my wife pull the fuel neck some so I could cleaned it.) It did not leak for a while until yesterday. With a full tank fill-up, I could see the fuel tank was wet with fuel around the clamp area again! (Naturally, there is gas smell in the garage.)
I removed two covers in the trunk to access fuel pump and tube connections area of the fuel tank, and found no fuel leak or wet spot there.
I called my Volvo dealer, a technician told me there is a gromment (gasket or O ring type of material) between the fuel filler neck and fuel tank, to prevent it from leaking (Volvo part #6842188 $13.51). My problem is that I did not see this gromment while I cleaned it twice. Would the gromment shrink after 8 years of service? If there was not a gromment before, why it was not leaking fuel before? Was this a real problem?
It seems that the fuel tank, fuel pump, fuel lines, etc. all have be removed to remove the fuel tank to install this gas tank gromment. Is it correct? Is there another way to fix it?
thanks and best regards,
Joe 1996 850 87K
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