I found a 1966 421 tripower that had been rebuilt in 1987. The engine was never used and stored properly. I took the engine to Boyd's in Norman, OK to have it cleaned and checked. The engine had Howard's aluminum rods and I wanted them replaced with new H-beam rods. They examined the 421 and advised the engine was a 4 bolt main 455 that was bored 30 over and was in great shape. However, the engine block code indicates it is a late 1966 421 block. The factory bore for the 421 is 4.09 while a factory 455 is 4.150. However, this block shows a bore of 4.180 which would be a 455 bored 30 over. Now, the shop is concerned about the overbore being too much. It is my understanding that the 389-455 blocks are the same block with different bores and stroke lengths. The shop has not yet advised me to use another block. Does anyone have a suggestion?