Chevy Small Block Bolts Question

[Keep it stiff]

I need to change a couple of water pump bolts to be longer and with allen key head to be able to accommodate an alternator bracket.

Can someone tell me please what the thread is? These imperial sizes are a whole new world to me! I think they might be 3/8th and if so UNF or UNC?

Thanks.

[Keep it stiff]

Thank you.