Are you sure the tank needs replacing? If there's crud in it, it can be drained out. Isn't there a drain plug (ie. screw) at the bottom of the tank? You can run the car till it's low on gas then drain. Looking through the in- tank-pump hole should give you an idea of what is in there.

Of course Safety is a concern when working with gas.

So, $100 does sound like a good price. I'd keep the old tank as spare if it isn't rusted out.
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'80..245..M46...b21a..SU carb..

If you're not used to handling fuel tanks, think it out carefully
SAFETY, SAFETY, SAFETY! We don't want to see you on CNN.
Simple things like dropping wrenches create sparks.
Ok I'll climb down off of my soapbox and stop yelling.

First, have a fire extinguisher handy, A garden hose with a trigger sprayer. Safetygoggles (if your cloths get fuel on them, soak with water immidiatly better to be soggy than on fire)It doesn't hurt to hose the driveway down as well ( the guy in the shop with it on a rack can jump back , that's a little tricky when you are on your back.)

Second you'll need an external transfer pump to pump all of the old fuel out into approved containers .
Actual tank removal should not be too difficult.
Soak the mounting bolts down with Aero-Kroil ( or P'blaster or goose grease of choice)the day before.
Disconnect battery first.
Disconnect all the hoses and filler neck.
Put a floor jack under the tankwith some plywood or board to even out the load.Remove the bolts and lower it down.

As soon as you get the old tank free, fill it with water. An empty fuel tank ( that is to say, empty of liquid fuel, but full of vapours) is more dangerous than a full one.

I'm not trying to scare you off, just inform in case you were unaware.
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-------Robert, '93 940t, '90 240 wagon, '84 240 diesel (she's sick) , '80 245 diesel

You say the tank is full? You can use some kind of pump or syphon to remove the gas, but dropping a full tank is dangerous and difficult (especially if you are by yourself) I dropped a half full tank on a station wagon with my dad and brother; it was no picnic. If you don't have the time (which you'll need to get the gas out) or the extra hands, i'd just pay the extra money. conveinence is a much underappreciated value.