Ron,

It is not recommended to plant a new tree on top of an old stump area as the sawdust/mulch resulting from the grinding has a high carbon to nitrogen ratio, which means that the new tree won't have much nitrogen to help it grow.  It is recommended that you develop a new planting hole at least three feet away from the stump.  This would allow for adequate minerals in the soil and provide enough growing/rooting space.  See AskExtension-Don'tPlantNewTreeAtopRecentlyGround...https://web.extension.illinois.edu>thisQuestion>Threadl

 Thuja Green Giants require a fertile, well-drained but moist soil that is protected from wind and PM sun.  You should fertilize each year with 2-4 lbs./sq. foot of slow release fertilizer or top dress the area around the plant each year with 1-2" of quality compost. While planting make sure the zone that separates the top of the plant and the root zone or flare is 2-4"above the soil line.  If your plants are in containers make sure the roots are cut and spread out before placing in the ground (not circling around the plant).  Water deeply every 2-3 weeks during periods of extended dry and hot weather.  See https://api.ag.purdue.edu>api>depotws>File.  A general overview of tree installation is covered at this website: http://www.extension.purdue.edu>FNR-433-W.

I wish you a fine-looking, healthy hedge!

I have spent a lot of time researching your question and can find no answer based on solid research through a university.  Therefore I suggest you contact the Morton Arboretum Plant Clinic in Chicago at <personal data hidden> or phoning at<personal data hidden> between 11am-3pm (Central time) on weekdays.  It might also be worth a phone call to the Holden Arboretum in Kirkland, OH, which in 2017 had a class on best practices for tree planting.  They may have someone on staff who could help you.  Their number is 440-946--4400.