[Casey B]

I have had to remove the dreaded fibre glass as well and have to say that the best method for me wast to use gasoline as it is not really designed to contain fuel. It does soften it to some degree. the other method already mentioned was chipping away Cut it first with a sharp knife and peel it away from the metal. 10 chances to one the person who did this didn't prep it properly so it should peel/break away.
The other tank that is stuck...probably just dirt and debris and a tight fit. If it is a tank with no folds the welded type aka the replacement tanks.they are heavier and a tighter fit in the tank well. Once it was out and all the debris removed and clean it was still a tight squeeze to put back in.
Good luck and let us know how you made out!
Casey