Bought my 93Z used and the former owner says he replaced the motor with an LT1 from an impala, is there any way to tell if this is true (casting #'s)?

Besides . . . what's the difference anyway?

 

The first and most obvious thing would be that the b-body engine would have iron heads (if he replaced the complete engine).

 

if he did another way is look for an extra knock sensor on the drivers side with nothing pluggged in it. besides the iron heads being heavier they do flow better than the f body heads (stock)

 

The b-body cam also produces slightly more peak torque at a lower rpm and peak hp at a lower rpm, the lowerends are the same though.
Also the iron head motor was the same accross the board for the Roadmaster the Fleetwood, the wagons and was standard in those cars it was optional in the Caprice be it civi or 9C1(copcar). Do not let the factory ratings lead you into believing that it is weak took at the torque and consider that factory configurations of intake and exhaust choke most motors, and the b-body ratings are setup for 87 octane reprogramming can pay off well with power gains as it can with f-body motors.