When I bought my 37, it had a hole cut in the right side of the body for an exteriior fill gas tank. The tank it came with fits under the seat just fine, however I don't know what year the tank is from. I'm assuming the 38 to 46 tanks are the same size. The person who did this modification also cut a corresponding area where the tank goes for the fill tube to exit the cab. I can take a picture of mine for you if you wish. Sorry I cant help you with the seat hinge part of your question.
