You bought the wrong studs. The ones with the 1/2-13 thread on the bottom are for converting press-in stud heads to screw-in studs, the empty press-in stud holes accept the 1/2-13 tap.
Buy a set made for a big-block Chevy, they will have the proper 7/16-14 thread on the bottom.
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1964 Tempest Coupe LS3/4L70E/3.42
1964 Le Mans Convertible 421 HO/TH350/3.08
2002 WS6 Convertible LS1/4L60E/3.23
1966 VW Type 2 Single Cab Pickup 140hp 2276
1956 VW Type 2 Panel Van 36hp 1200
1957 VW Type 1 Beetle 180hp 2276
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