trying to unload this old van and would like to know what engine it has, never owned a dodge, where might i find the engine casting numbers on this?

 

You can get that info from the VIN, or from a sticker under the hood, on the radiator support bulkhead.

To distinguish a small block from a big block. look for the distributor. If in at the rear of the intake manifold it's a small block (Most likely a small block, in 78 it will probably be a 318 or a 360). If the distributor is at the front of the intake manifold it a big block (again, in 78 it would have been either a 400 or a 440).