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Black iron tanks, such as found in the Allied Mistress, can be easily repaired by building an epoxy fiberglass liner 'inside' the existing tank, using the existing tank as the 'mold' for the fiberglass 'liner'.
For such a rebuild, you will need to cut large access holes in the top that are 'wide enough' to get inside to: extract the built-up gums and biological deposits (WEST recommends using garage floor 'degreasers' that 'absorb' the oil, etc.), then surface sanding/grinding to get the metal back to bright and with a significant amount of 'tooth' or surface roughness to the base metal. Then several layers of 4" wide FRG tape is applied across all weld zones (to reinforce the 'usual' places of leakage), followed by applying cloth to build up to ~3/16"-1/4" thick FRG for a nominal 100 gal tank and finishing with a resin-rich application. The large access holes can be closed using large (11 gage, etc.) plates that are applied to a 'studded' closure configuration (bolts affixed to the underside of the 'cut' and penetrating through the old tank wall ... you can do all of this 'sistering' by using a portable drill - drill, tap, screw-in, etc. simultaneous through the new access plate cover and the edges of the old tank). Gasketing for the new closures should be precut 'strips' of neoprene/Buna and can be obtained from the usual online industrial supply websites, then caulked/sealed with fuel compatible caulk or 'gasket compound' (Permatex).
Such is almost as durable as a new tank, doesnt require the removal of the old tank, and is vastly cheaper (your time is 'free') than a new tank. Only special consideration is that the liner be completely free of pin holes, etc. as fuel (& water and microorganisms) leaking between the FRG and the old tank walls will 'accelerate' any corrosion. Some tank builders will use a 'spark test' to validate the integrity of any installed 'liner'.
The WEST SYSTEM 'projects' website has quite a few 'fuel tank liners' illustrated and explained. Such 'FRG liners' should NOT be used for water tanks ... due to the organic emissions from the FRG.
hope this helps.