Holley/Demon Carb stumble

If you can find another one, or a primary off a 4779, (ie 750 DP), or one off of a 4412 500 2bbl you'd be set as they are all the same as best as I have measured them. I'd be willing to bet the 4780, 4781 are the same too.
You do have the rear powervalve blocked off right? And are running jet extensions on the secondary side?