I am needing to replace the front tank on my short box 1989 F150 since it will only hold about a 1/4 tank, if it is any fuller it leaks all over the ground.

What is the best way to tackle that? Remove the bed, or is it simple enough to do from underneath? I haven't tackled a tank project just yet.

 

If you can easily pull the bed, it helps. But you'll have to drop it out the bottom either way to remvoe the tank.

 

You cant physically remove the tank from the top. There is a cross member and the truck cab you need to get past. Ive dropped mine from below. If you have the 4x4, you'll probably need to remove the skid plate, but it's pretty easy. Throw some penetrating oil on the strap nuts the night before you want to drop it.

 

I did the first one on my former '97 F250 by dropping it down - not enough slack in the hoses to get the fuel-line disconnect tools up there..., reaching around the tank...I butchered up one of the fittings and ended up needing a new complete new fuel line (now obsolete), so BE CAREFUL with those fittings!
On my current '95 F250, it was MUCH easier to lift the bed off the frame for easy access to the fittings/bolts. It helps to have a buddy with a lift, LOL! . I did BOTH pumps/sending units, along with a new front tank, not because of rust through, but rust on the INside of the front tank from sitting for years with an inop pump before I bought the truck... Easier to replace the tank than trying to clean all of that rust and 'turpentine-smelling' bad gas. While I was at it, I also did the Fuel filter, added new frame receiver hitch, powerwashed the frame inside and out.... All that in less than five hours! Of course it also helped that this was a rust-free Florida truck, not a northern 'rust-bucket'

 

I removed the bed on my '88 the first time, but the second time, I lowered the tank.
When I removed the bed, the original nuts on the bed bolts wouldn't budge.
I ground the heads off the 4 rear bolts and cut the front two bolts with a sawzall, cutting between the frame and the bed cross member.
The fuel line fittings are the easy plastic clip style, through '89.
Have a plan if you're going to remove the bed, as to where you're going to set it down. I wrinkled my left lower quarter panel, by setting it on the rear bumper.
I made a grid that fit under the top bed rails, using two pieces of 7/8" unistrut and two pieces of 1 1/2" electrical conduit, then lifted the bed by attaching the front edge of my tractor bucket to two 1/2" beam clamps.
The quick release friction style cargo straps work good for lowering the tank. I used one strap near each factory strap, then lowered it a little at a time, the second time around.
You also have EVAP lines which are attached to rubber grommets to remove.
I did mine outside, so fewer sources of ignition, for safety purposes. I don't have a garage, so it wasn't an option anyway.

 

I don’t think I’ve ever seen a skid plate on one of these trucks. Definitely use some penetrating oil and let it soak.

Not applicable. The OP’s truck has plastic retaining clips on the fuel lines.

Lower the tank a little bit and then all you need is a small (pocket) screwdriver to remove the clip.

When I removed the bed for the frame swap I had to do that for every bed bolt. What a giant pain in the rear. All I have ever used in my gravel driveway is a floor jack.

 

I have done the front tank from below twice. First time was to replace tank and fuel module. Second time was to replace defective fuel module. I was able to remove and install by myself both tanks on my truck in a day. Not the best way to spend a day, but it can be done. Tank replacement definitely from below. Fuel modules can be done by removing the truck bed and going from above. I did not attempt to remove my bed because I have a lift gate on it. If at all possible, purchase original Motorcraft modules. My first go around was Spectra Performance tanks with one Spectra Performance fuel module and the was purchased from LMR. About a year later, both had bad pressure. I don't run my tanks below 1/4 on anything I drive. The fuel gauge goes to 5/8 and I usually top off tanks and use premium instead of regular unleaded. As far as the tanks go, compare them to the originals. Some metal persuading may need to be done. Sometimes the flanges need a little trimming and a little more bending to fit more true. I would purchase the Spectra Performance tanks again, unless I found new old stock for cheap.