Another bolt size needed

[Lippyp]

Sorry guys, yet again I am struggling with finding a bolt size. Can anybody tell me what size the engine mount bolt (the big single one that goes from the tower on the frame to the rubber mount) is on a 69 350 SBC. I had to take the entire tower out to remove an old bracket whilst changing out my P/S pump at the weekend and the old motor mount bolt is looking pretty chewed up and the rubber is a little old and covered in paint overspray, so I am going to replace the mount and need a new bolt. I have found a good supplier for UNC bolts now in the UK but need to know what size it is.

Thanks