What is the transmission oil type and capacity by volume required?

Could you verify the engine and year please? The 3.5 wasn't used in 1999, it wasn't installed in a van until 2013 and that is in a Transit.
A V6 that was used in 1999 was the 4.2L. That engine uses 5W-20 and 6 quarts with a filter change. However, it is better to add 5 quarts and then check it, then add the rest while checking the dip stick. As for the transmission both of the ones used in that year take Mercon V ATF fluid, The 4R70 holds 13.8 quarts while the 4R100 takes 15.9 quarts. In both cases that is the amount used in an empty transmission and converter. You need to remove the pan to change the fluid and filter. Remove the bolts circled in red, then loosen the blue ones and let the pan drop onto them to start draining the fluid. Then hold the pan and remove the remaining bolts. Check the magnet for metal and debris, there will be some on it from normal wear. Clean it up for reinstall. Remove the converter access plug and then rotate the engine with a bar until you can access the drain plug on the converter. Remove that and drain that out. Once fully drained reinstall the new converter drain plug and torque it to 22 ft lb. Now replace the cover and tighten it to 9 ft lb. Replace the transmission filter and reinstall the pan. You can reuse the gasket provided it is not damaged. Torque the pan bolts down using an alternating X pattern. Now add at least 5 quarts of fluid and start the engine for a few seconds and shift through all the gears. Shut it off and add more fluid and start the engine and shift through the gears. Then check the fluid level using the dip stick.

https://www.2carpros.com/articles/how-to-change-engine-oil-and-filter
https://www.2carpros.com/articles/how-to-service-an-automatic-transmission