400 low buck dont give a

If you build anything with the "low buck don't give a ****." attitude then you'll get what you get. I don't think the combo you have will spin tires on a wet lawn! Lol!
But seriously, I'd lose the 750 double pumper for a 750 vacuum secondary & go with a milder dual pattern cam & dual plane intake. Unless you really don't give a **** & just want it to sound cool then throw together what you have & have fun with it.
Carb size does matter in a build as does the way the secondaries are operated. Too big a carb & you'll lose efficiency.