What size bolt fits holes in rear of L31 Vortec heads?

[L31 Vortec swap]

What size bolt fits the holes in the rear of the vortec heads? These are the bolt holes that face the firewall, and I assume they are for accessories.

Thanks in advance for your help---access issues have left me uncertain and prevent an easy test fit, so the info will save me time and scraped knuckles!

 

[L31 Vortec swap]

Sorry about posting in the wrong subforum. Reposted in the main forum.

 

[FastOrange]

3/8.

Takes a 9/16 wrench

 

[SixSpeedSS]

Keep in mind these holes are exactly the same as the front holes - and in the same position. The cylinder heads will fit Left or Right (meaning they are identical).