[storm9c1]

FWIW, I do know you can go the other way though. I am running an engine in a project that is a 1-piece block with a 2-piece real seal adapter. The reason for doing this is if you have a special crank (perhaps a custom internally balanced crank in a stroker for example) and you want to cheaply run it in a modern roller/vortec block. A high quality adapter won't leak and makes it easier to change the 2-piece seal if needed. Not saying running one of these is a great idea. Switching a 1-piece for a 2-piece is going backwards, but it does work if you need it for some reason. It used to be an old school (well late 80s & 90s) hot rod trick since it will expand your options if you want an internally balanced crank on a 1-piece block. Normally a 1-piece block eliminates some (or all) of the rear counterweight on the crank thus often forcing an external balance crank on 99% of the 1-piece projects. That's a little more history as to why it's hard to switch from one to the other.

Nowadays there are many more crank choices available, so it isn't as common to do the 1-piece to 2-piece conversion.