overtorquing? not if you set your wrench correctly :)
a builders cube is essentially a small radiator stop leak cube made to bind up and plug a minor leak in a gasket . It's not ideal.. but it's a lil insurance.. and it's not a huge bottle of plugging goop like them bottles of bars leaks. the ones I've used were like 2 'sugar cube' sized blocks. I know lots of shops would always toss in a couple oin a new rebuild... etc.