eee that one's pretty stretched. Some you can straighten, some are really badly stretched/deformed & oblong and the metal is too stretched to straighten by pinching. Have fun digging that out of the average Orie-gone gas station attendant JY 250-300K mile hoopt...
It's not too miserable, but it's in violation of 3 of my repair rules, and thus I'd avoid it like the plague;
1. Under the car.
2. Dirty.
3.Lots of a toxic liquid (coolant or gas) to potentially spill.
4. Heavy lifting/heavy work/actual effort/potential for injury or whatever physical malaise may be possible or probable.
Removal of the entire tank isn't required, but a TON of dirt sits on top of the tank from years of road use that will want to go everywhere as you try to disturb the band clamp, and you don't want dirt on the o-ring/in the tank when you replace it as petiww says.
But you took the greazy nasty mess of the engine (fluids, filth n' all) out and then did the heater fan second, this should be just the thing!
Try a better cap off an 850 or something first (easy), but if it's spilling out on turns, you probably need a new filler tube. Also check the evap hose on top of the filler tube just in case it's rotten or something (won't cause the exterior spill, but will cause a leak in the car).
Early cars and diesels don't have the problem so badly (steel on early cars), diesel doesn't have the narrow spout and the nozzles on diesel pumps aren't as thin and brutal. Diesel has no provisions for the evap system either though heh.