Hi Simso
If I was you I would try to save it for those kinds of dollars
If you fail you can spend money afterwards
I grew up without any budget to work on these trucks
So I feel your pain

use the nastiest most destructive paint stripper to remove the old lining that is failing, the stuff that comes in metal cans and melts nitrile gloves
After you slowly turn the tank every way to let it sit for ten
Minutes in all surfaces the liner should be like thick gravy skin

Put a lot of gravel in there the shake it all about with the paint stripper if you can
Repeat the stripper and gravel until it’s reasonable
Have a plan where to dump that stuff before you start
Some metal buckets use a plastic coating on the metal
That coating is rolled into the bottom seam of the bucket
That will melt and the bucket will leak
Suggest a metal bucket for slaking lime,
Those are usually soldered tight

Let it dry good and maybe even do the gravel shake dry too and see if any more flakes out

Heat the edges of the kink where it is bulged to push and massage it back into shape

When you get to “welding” I suggest don’t
Use braising or silver solder with decent overlap in your patches
Maybe even use small screws or fold crimp the patch and braise the fold for a liquid tight seal
Oxygen/ propane works very well for brazing and silver solder btw

Also check the original outside seam if you find leaks braise away

Don’t bother with relining the tank
Modern fuels that contain ethanol won’t allow water pooling and its always a loosing battle in the long run because moisture rots metal under the lining anyway
Just plan on using a fuel filter

I’ve got gravel cleaned rusted tanks going onto 15 years now no problem in Canada
One doesn’t even have a fuel filter
If you use them enough it will be okay
-s