Thrust Bearing Shims:

From:
https://forums.maxperformanceinc.com/forums/showthread.php?t=795295&highlight=455+crank+400+block

Excerpt from post:

61-63:
"Yes, you use 455/425/400 olds thrust bearings on 389/421-428-455 conversion; Mahle BC304J, Clevite 804P, and cut half moon shims out of .020 stock (I think, measure to be sure?) which are placed under the edge of the thrust bearings when they are placed in the block and cap, and are captured there once installed. The Olds 3" mains had the same width in the thrust bearing as did the 421/428/455 Pontiac.

Pontiac stepped up to 3 1/4" mains so there would be some overlap in the throws from the main and rod journals. Way over-engineered. Ford 427s only have 2 3/4" mains. Smokey Yunick cut the 421 mains down to 3" for NASCAR because the larger mains generated too much heat and the engines would not live through a 500 mile race."

Below Pics show the shims and how they look installed:
(not sure where I got the pics, probably from PY and/or Pontiac Dude?)