There are 2 flexplates that will work on both 350 (and all pre 86 V-8's) and the 230/250 6 cylinder engines. One is a 168 tooth and the other is a 153 tooth which is smaller. You may have gotten one from a later 350 (86 up) which will not work as it has a different bolt pattern.
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