Technical Generator bracket bolt size

Hello everyone. I am trying to install the generator brackets on my 1956 Customline with 272 but I do not have the bolt that goes in below the exhaust manifold and into the side of the block near the oil dipstick. Does anyone happen to know the size of that bolt? I went through all of my spare bolts and don't have anything to match it unfortunately. Thanks for your help.

 

Not a whole lot of choices for that one. 3/8, 7/16, 1/2 or and that would be damned big 5/8 all with course threads. You weren't digging in the metric bolt bucket?

 

3/8" co**** 16? not sure on length easy to check the depth I think 1" been a while

 

Thanks for the replies. I will head over to my local hardware store and just grab a few of each since I am sure they will come in handy to have around either way.