How to clean out a crusty old gas tank?

I have acquired another old tractor. I tell myself it's a rescue. It's been sitting a long time and the fuel tank is pretty gross with rust and crud build up. Its keeps clogging the line. Replacement tanks don't appear to be readily available, I haven't contacted a dealer for OEM but I know I wont want to spend dealer money for one. Do you guys have a preferred method for tank cleaning?

Also I wanted to run an idea by you guys. The fuel feed is just a hole in the bottom of the tank, here's a pic

You see that reducer between the tank and the shutoff? My theory is to replace that reducer with one that has threads all the way through (I will probably have to make it) and put like a 1 inch long pipe nipple in the top side that will stick up into the tank. That way any rust or debris in the bottom of the tank can't just slide right into the line like it can now. I realize that I will lose some fuel capacity because I wont be able to run the tank completely empty but I'm ok with that if keeps the crud out of the line. What do you think of this plan?

That would probably work as long as none of the trash was light enough to get suspended when the tank sloshes. 

How hard would it be to pull the tank? The last one we cleaned, we pulled, dumped a cup of pea gravel and some soap and water in it and took turns rocking it back and forth until it was clean. 

Can you pull the tank?

There's a DIY method to acid wash the tank with muriatic acid to get all the crud out. I'd then flush it and coat it with one of those epoxy fuel tank kits, which are freaking awesome.

In reply to The0retical :

Thank you! And big thanks to Jumper for the excellent and concise write up. This is exactly what I needed. 

I think I will do both methods mentioned above, first using the aggregate method to get the heavier build up, then the acid and sealer to really do a nice job finishing it off.

The0retical said:

Can you pull the tank?

There's a DIY method to acid wash the tank with muriatic acid to get all the crud out. I'd then flush it and coat it with one of those epoxy fuel tank kits, which are freaking awesome.

^^ This. I did this on my old 77 Celica with a kit sold on Amazon for $70(POR15 product), included the acid bath and a sealant to coat it with afterward...worked like a charm, but was a labor intensive task(turning it all around, shaking it above my head, etc....can be kinda heavy if doing it in 100 degree+ heat like I did.

I just did a rusty motorcycle tank,the $$$ kit for the job didn't really do the job.

 So I used about 8 litres of cheap toilet bowl cleaner,left for a couple days swishing it now and again,was working but I'm impatient so I topped the tank off with vinegar.

 Left for a couple days,came out very nice after wards,dumped some 3 in 1 oil and swished around to keep it from flash rusting.

 Looks good still.

I cleaned out a motorcycle tank with muratic acid, flushed with water and sprayed WD40 inside so it would flash rust. More than a year later it is going good, no problems. I didn't use an epoxy liner as I have seen issues with it coming off and plugging the fuel system, plus the tank had a small steel line for the vent that was a bugger to clear out and didn't want the epoxy to plug it.

Be skeptical of the liners. What used to work seems to have separation issues with today's fuels.

Pete

Yeah, I'm starting to reconsider the liner. I don't have any leaks so might skip that for now.