I can not figure out why a so called original replacement sender has 1/2-18 threads and no bushing found for that to NPT? VERY odd. It almost seems like the cast iron intake had 1/2-18?
Buckshot should be able to give us an idea of thread size in cast iron and sender PN from his 57...... or a pic or two. i know that is a bit moot with Van's Edelbrock but we need some history on this weird thread or a bushing PN.

Also Van, what is the part numbers of senders you tried and what thread is on them and what size is in the aluminum manifold ?

Right is right, even if everyone is against it, and wrong is wrong, even if everyone is for it. - William Penn