Cleaning plastic fuel tank

[mmwb]

I recently dropped the fuel tank (plastic tank) on my 1975 M880. It had sat for some time before I bought it. I would guess that it had between 5 and 10 gallons of the nastiest, brownish orange, sour gas you could imagine. I got most of it out, but it probably has a pint or so left.

Any tricks to get the dregs out?

Is there a method to get the tank clean (remember this tank is plastic)? The sending unit is nice and sticky with the varnishy residue we so don't want in our fuel system...

Also, most tanks I've seen have some sort of pad between the straps and the tank itself. This one had the straps riding right on the tank. Is this normal?

Responses will be appreciated!