Bite the bullet and do this the right way. Determine if the bulkhead removal
would actually give you all the room you need to get the old one out. If so plan on going that route
. Because tank issues crop up from time to time take this opportunity to make it more accessible in the future. Manufacturers mostly do things for their convenience and not ours down the road.
or borrow a Fein reciprocating saw that allows you to cut through the bulkhead with out destroying it. (Get plenty of blades - couple types - straight plunge cuts and circular for going along a line. If done properly it can be replaced with the addition of a little trim around it and then if you need to go back in later it is just a simple matter to unscrew the panel and good to go. Leave yourself an inch or so all the way around the bulkhead for the trim to screw back onto.
Over the years I have replaced both 100 gallon diesel tanks
and added on as well on my power boat
. You may decide to put a fuel
gauge in it while you are at it. From time to time the hoses need to be replaced due to rot
etc so getting access now may be helpful again.