Depends on whether or not they used the engine number as an ID (vin) on the title to the truck in the state it is registered in. Some states you may have to prove you did an angine swap and change the numbers on the title.

Need to talk with the local DMV if there's any doubt.

Others will chime in that can tell you where to look for the numbers. One place will be a flat machined surface right beside the distributor. There may also be some numbers cast in place on the block around the starter area.