[84 400]

1406 eddy is the standard for most folks and work great right out of the box. Pretty much any 600-750cfm carb will work well.

There are square bore and spread bore intake manifolds and carbs. Instead of running the adapter I would swap on the matching intake at the same time. Less fail points that way and if nothing else it will cut some weight.

On the C3 corvette forms they are all into this cat named Lars. He is a q jet guru and will rebuild your carb and set it to your apeca. He also has great reference papers to help you set up your stock carb better then factory. ”The Lars papers ” check them out. Also some great timing info from the on him.

Ultimately a well tuned and properly art up q jet will get better MPG and could outpreform the eddy. They are not well understood carbs.

Here is a link to some of his papers and his contact info. No affilation on my end but the better and z28 guys swear by his setups.
https://nastyz28.com/threads/lars-qjet-papers.183265/