** is there any problem with building a shelf where the range hood was to hold up the microwave? **
Probably not but it may require extra cleaning as it will likely attract grease from what is being cooked on the stove. That grease can also get inside the unit if used while cooking with the surface burners.
BTW. There are ready made microwave shelves available for such an application. You can see one such unit at the following link:
Microwave Oven Shelf
** It would have 6" clearance on either side of it - is that adequate for venting? **
You'll have to look at your model to determine where its venting is (slots in the outer rapper possibly on the top, sides and rear). Those areas will need to have sufficient space surrounding them. 6" should be fine I would think.
The Appliance Information Site
[This message has been edited by DanO (edited March 02, 2003).]