The 3" deep board is more for the 5108 for 60mm and shorter focal lengths - the 1" deep board is fine for the 504 and a 40-50mm etc. The deep board is a bit of a pain to change lens settings on, so only use it if necessary. The extended boards are really more for 300mm's etc on the 5108, copying applications, shuttered lenses etc.
I'd suggest getting a 1" recessed L39 thread (1972), a flat L39 board (1973) and if you need it, a flat 50mm x 0.75mm board (1974). Obviously, you may want to get multiples thereof.