We're putting a ZZ4 into a 65 Chevelle. The gas tank hasn't had gas in it for over 10 years. Took tank off and can see orange grit on bottom of tank through sender hole. What is the best way to get it thourghly clean? Tried garden hose and nozzle but not enough pressure.

You may not want to hear this but the best way to handle this is to take the tank to a gas tank shop and have it cleaned and sealed. There is no way to do this at home that is both thorough AND SAFE. Considering the cost and the amount of damage that can be done to your engine and carburetor this is cheap insurance.

Steve