With a gas tank assembly you have the tank, sending unit, wiring, and gas tank straps, not to mention nuts and bolts which could all be replaced but would any of the new parts be better than what you have ?.
I've read and replied to many threads about persons replacing a gas tank and some have reused combinations of old and new parts while others have bought all new parts only to have problems. Sometimes things go smooth and persons reusing some of the old parts have not had problems but others have been in a nightmare.
Unless you have a lot of time to burn and possibly extra money, look and test what you have and get an idea what kind of shape it's in and then weigh out if you would want to reuse an old part or replace it with a new one.
In a perfect world all replacement parts work as designed but things are not that way.
If I had a new sending unit I would get out my meter and make sure it works as it should as far as the resistance measurments with the float arm at the full up position, full down position, and then possibly do some checks with it a quarter of the way up, halfway up, and three quarters of the way up. Doing a simple test like this allows you to know, or be more sure later, that if there is a problem at least this was checked and can be pretty much taken out of the equation for some other troubleshooting if needed later.
Jim