Mash tuns should be wider than deeper, I think, about 1.5:1 width to height. Kettles about 1:1 ratio. Make sure your slope is significant enough in case the floor oil-cans when it heats up. My kettle has a slope of 3/4" per foot draining to one side and the center sank enough after use that I can never get all the liquid out.
I use a plastic shovel to scoop out the grain, but then mine is just 2bbl.
First batch we filled the mash tun to the rim for a big porter. Go bigger by 30-50%.
For mashing in a lot of pro-brewers just use a canoe paddle to mix. No big deal unless you have shoulder problems.
What are you using for the false bottom screen?