[bergy]

I just went through that with my '70 Boss 9 Tony. The car had the original tank that was in great exterior condition (after I removed 1/4" of undercoating).
One of the restoration guys may have a better answer, but here's what I did:

1. Drained the tank & removed it.
2. There was lots of semi loose crap stuck to the entire interior. So I rinsed it out good with degreaser & let it dry.
3. Put a couple of pounds of 1" clean gravel in the tank and shook it all around for about 10 minutes. This knocked off all of the loose stuff inside of the tank.
4. Emptied out the gravel & washed & rinsed the inside of the tank.
5. Used Eastwood's 3 step tank sealing system.
6. Put in a new sending unit & good to go.