The offset bolt pattern mounts to the block and fits the 168 tooth flywheel/flexplate. The straight across bolt pattern mounts to the
block and fits the 153 tooth flywheel/flexplate.
Your original manual transmission has a 3 hole starter that mounted to the bellhousing and matched up with a 168 tooth flywheel.
Fred

1956 3100 Pickup/Red/350/3sp OD/PS/Disc Brakes
1957 Bel Air Sport Coupe/Red/355/TH350/PS/Disc Brakes
2017 Silverado LT Single Cab SB/Black/5.3/6 Speed Trans
1947 Willys CJ2A w/F-Head engine