Not true. The diesels had a unique block and other parts.
From Olds-Diesel.com:
"The 350 Diesel is a Converted 350 Oldsmobile Gas engine Right?
Wrong! The 350 Diesel Is not a converted Gas engine..it is a completly different block..It isn't a gas engine with different heads or a gas engine sleeved, etc etc..There was no gas Oldsmobile engine sharing the same block. The Diesel block is specific to the diesel, it is a heftier, more solid block with stronger mains and a port in the valley for the injection pump adapter. The block is cast with a high nickle content. The 350 Diesel is not a converted gas block but can however be converted into a gas engine."