I would always run 10-40 in mine. A good tune would be new Plugs, Wires (if needed), Points and Condenser, Adjust the Timing a little Advanced (as long as it doesn't ping), clean the Carb (spray clean with it running should do) and adjust the Carb to the Highest Vacuum. Also your PCV Valve, Air Filter, Fuel Filter, etc.
The GVW of 19,500 is a little low for a C-60. Are you sure it's not a C-50? If you do go with a Detroit and heavier trans someday, you will lose about another 1000 Lbs off of your hauling capacity if you want to stay under the legal limits.
Keep us posted on your progress.
