Having recently played around with both of these engines I thought I'd post some pictures to illustrate the differences between the 389 blocks (produced from '59 -'66) and 421 blocks (produced from '61 -'66). Both blocks shown are of 1964 vintage. 389 block. 421 block, note the flat machined face on the passenger side just below where the head bolts on. 389 block, no transfer lug. 421 block has the transfer lug. This lug signaled the line workers to transfer the 421 blocks to a different machining center, the 326 blocks had one as well. The far more common 389 blocks didn't have the the transfer lug cast into the block. 389 block with 2-bolt 3.00" diameter mains to accept the factory 3.750" stroke crankshaft. All were equipped with 2-bolt mains with the exception of the rare '59 -'62 420A, 425A and Super Duty 389s. All of the famous '64 -'66 GTO 389 engines were 2-bolt main engines. The somewhat peculiar-looking oil baffle/windage tray and dipstick tube arrangement that attaches to the #4 & #5 main caps is a feature specific to '64 and earlier blocks. 421 block with 4-bolt 3.25" diameter mains to accept the factory 4.00" stroke crankshaft. All 421 blocks without exception were factory equipped with 4-bolt main caps. 389 block casting (4.0625" standard bore size), note the scalloped side of the block at the deck where it mates with the head. 421 block casting (4.09375" standard bore size) is nearly straight across at the deck. There's a lot more metal in the casting where the side of the block at the deck meets up with the head. More iron is cast at the top of the deck near the valley area as well. 389 block, lifter bore area. 421 block lifter bore area. The three stiffening ribs connecting the lifter bores across the center are thicker. 389 lifter bores. 421 lifter bores, considerably thicker and more robust. Pretty surprising how much more metal was used in these 421 blocks compared to the common 389 blocks.