Starter bolt size is 3/8"-16
Check out our high torque mini starters on our website that include the bolts:
GM SB and BB Compatible Starter – 3 HP – 153 or 168 Tooth Flywheel – Tilton Style - Black
GM SB and BB Compatible Starter – 3 HP – 153 or 168 Tooth Flywheel–Tilton Style-Chrome
GM SB and BB Compatible Starter – 168-Tooth Flywheel – Denso Style - OEM Finish