I pulled the rivets off my T/A intake around 78/79 prior to installing it during a rebuild as they and the plate were loose, and tapped the manifold for bolts, as well as set the carbs up for studs... IMHO I think the original 340 manifolds had casting issues due to the alloy used, the aluminum was just way too soft and porous, I've refurbed a good number of six pack manifolds/carbs (sm/Bblk) over the last 3-4 decades, and factory T/A manifolds are always a mess in regards to the various fasteners and their threaded holes as well as water tubes/threaded ports and oil splash shields/rivets needing thread repair/inserts/modifications and in worse cases welding/rebuilding