I would start with 8 (or ten) foot runs of 3/16" beer line. You will have nothing but foam with 5/16". If you start with 8' you can cut off a foot at at time if the flow is too slow. It has been proven to be preferred on the slow side unless you are in a busy bar situation. The soda, which is usually around 30-35 psi, will need more length, I'd do a search but I think I remember 15 foot to be the length. There are some tricks to add restriction other ways like using 1/8" ID line on the inside of kegs, and also some mixing tubes, which you can search for.
You want your coffin to be sealed, with the exception of course being the tower that feeds it. Leave enough room for a 1" hose to blow cold air up there to cool the faucets and lines. It will return back around the lines. This helps circulate air in the freezer as well. You'll need some sort of trap door in the coffin to access the shanks etc. for maintenance. Insulate the coffin as best you can, the foam panels work well. Check out the show us your kegerator thread to see all kinds of innovative ideas.