If you were really going to pull out the 260 and do an engine swap than who cares about pursuing a small gripe over a ruined engine you were supposedly going to throw in the trash? There is nothing you can do with the 260 engine except wait for all that oil to evaporate for a few weeks put the correct plugs back in dump some fuel system cleaner booster in the car and see if it fires back up / burns off... if it doesn't then you will have to just admit defeat and pull the engine. Do you really think the shop is going to pay out 'damages' to an early 80s carburated engine of undetermined mileage with an ambiguous value somewhere below the $350 dollar range? Even if they had came to your house with hammers and destroyed the engine in the middle of the night on purpose it is still worth so little money their vandalism would not even qualify as a felony... How impatient still do you think a small claims court would be over a matter so financially trivial? Only the smallest claims courts in the entire country would even hear this case at all ... the court costs would probably exceed 50% of the value of the engine - if it had been in mint condition which it probably was not , based on your description... When I was standing over the 307 with spark plugs the other day I had a bad feeling mine were the wrong size so I put them back in the box and walked away. That's what you should have done too - b/c the parts store is just a vendor and not a GM dealership. Also, most parts stores are actually 'limited liability corp' s if you look them up so any attempt to collect a claim would be thrown out automatically in that case. My advice would be to definitely not try starting the 260 up for at least 2 weeks and leave the hood open in the garage until some of that oil evaporates before you try anything else or you could make it even worse now