Diesel engine flush

[Charlie's Bus]

1st post I have ever made here. I have a 1990 International School bus with a 7.3..... Jasper engine has 68,000 miles on it. Installed in 2007. The bus did not move for the last 2 years. I drove it 750 miles and change the oil. Here is my problem A very thick sludge came out. Do I flush the engine or repeat oil changes every 100 miles for the next 300/400 miles. Of I flush what do I use. The bus is being turned into a homemade RV. Help please....