he better have some paperwork that proves what it is. and that paperwork better have a block number on it that matches the engine. and that paperwork better also have a phone number of the place that did the work so you can call and verify with them what's what.
and I think you can pop off the oil pan and check the crankshaft to check of it is from a 4.2, though I've forgotten what you'd be looking for. :( part number maybe?