Bill,
These are 1850, 600's which have been modified. They now flow 654 each.
They are very common and Summit etc sells them. MANY specialty carb tuners and builders sell modified vacuum secondary carbs which will fit and it's easy to spend more money than your application warrants.
Depending on how stout your 428 is, the standard 1850 may be more than enough. A big cam and improved heads changes that.