You do not have to drop the tank. It is kind of tricky to get the pump in with the tank in place, but it can definitely be done. You'll have little problem getting the old one out. Use a brass or aluminum punch on the clamping ring. You may need more than one length of punch to get it, because the axle is in the way, but that part is easy.
I would also advise getting a new O-ring for the sender unit. The old one is probably flattened, and may also have rust and crud on it. Make sure that when you get the old unit out, you also get the rubber plug that goes on the bottom end of it. Make sure that plug is on it, and stays on it, when you insert the unit again. When you reinstall it it may be rather difficult to get the whole thing seated right. That rubber plug seats in a fitted recess in the bottom of the tank. You have to put it in a little high, then kind of tilt it in and down. You'll know when it's right, because the flange will obviously fit correctly in the hole. It may take several tries. Don't force. If it doesn't line up right, it's not in right. It's harder if there's gas left in the tank. It may also take more than one try to get the (*****ing) O-ring seated correctly behind the sender flange, and all three ears of the locking ring engaged at once. A little grease here helps.
Having done it once with the tank in, I have promised that next time I do it I'll drop the tank. With the tank on the ground, you can get the sender seated in about 20 seconds. If you live in a rust-prone area, it's probably not a bad idea to drop the tank every few years to clean off the accumulated salty mud that eats holes in the top.
