Magnum block have a crank sensor mount at the rear of the block on the passenger side kind of near the top of the bell housing. Yes they have cast into the side a number and in there will be either 318 or 360.

Water passages are the same and oiling is a little different, they oil the rockers from the pushrod instead of through the shaft. To tell if it has magnu heads look for the verticle intake bolts instead of nearly horizontal ones on an LA. However, that can be changed through re-drilling and so can the valve covers, simply by putting 5 bolt covers on magnu heads because the 5 holes still line up with 5 of the ten on a magnum.