Found a 6 gal Mercury tank that matches up perfect with a 9.8 Mercury I have. The tank has a  light coat of corrosion in it...NO flaking at all.. just light surface stuff.

What would be the best approach to cleaning this tank out.  thanks

If you feel you really need to use a metal tank, put a couple of pounds of small, rough gravel along with about a 1/2 gallon of gas in it and spend a bunch of time shaking the stuff around inside it.

Yes I feel that metal tank need....it fits behind the seat perfectly!.

Thanks for your tip.  Should I remove the fuel guage/line connector before filling it up with rocks and such and doing all the shaking?

Might want to take the fuel guage out, if it works.  Sometimes all the shaking can damage the guage.