Today while cleaning up my nephew's engine which is a 1988 350 roller block and it's face is the classic high low arrangement. I'm not sure what year they changed it.
When I did my vortec build years ago Northern Auto Parts sent me a timing chain set in place of the Cloyes "Vortec" set as it was on back order and it was a Milodon double roller unit for OEM roller cam blocks. After installation was when I noticed it was making contact with the block. When I called back Northern the guy says "Says here not for VIN R" which is a Vortec block.