I would like to state for everyone’s behalf upon this matter that this is action is a disgrace. These cars are legendary for reliability. If you drop a small block into it you will lose reliability, and as you sit alongside the road with your broken small block, you will eat away at this beauty that this car earned. To answer you question 528lbs is not unreasonable at all. This is a diesel engine and diesel engines weigh more than that of a gas. Look at a Cummins, Power Stroke, Detroit, or whatever descent diesel you like to. A diesel engine must weigh more due to the high compression. In these engines case about twice as much as a normal gas engine.
Furthermore a small block with cast iron heads and block comes in at about 600lbs. I know this due to the fact I was considering swapping out the engine in my rattlebox corvette to a big block.
Please get a GM product to hold this small block.