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Well my cataloging system failed me as I have lost bolt numbered 397-1. Of course, it is item #5 in the AIM that has the referenced part number cut off by the terrible scan job.
Does anyone know this bolt size? Even better - anyone know the part #?
Are you worried about it being the same head marked bolt that GM used ...or just one that will deserve its purpose???
I would imagine the bolt is a 3/8-16...but it also could be a 5/16-18...and IF you noticed in what you posted. The EXACT same bolt is used for the side cover. If you still have one...use it to compare to due to all of the bolts are the same thread..
This can be easily picked up at a hardware store....if you are not worried about the head marking.
I just looked in my 69 AIM and #5 says, "Part of trans asm". There is no part number.
From this info, I would say you should use the same size bolt as the other cover bolts on the forward bracket - 5/16-18 X ¾". The rear bolt should be the same size and the other 3 tail housing bolts. All I could find on that is 7/16-14 -no length given.