1) use the brand carburator you or a friend likes to work on...whats 'best' is usually very personal taste
2) 750 imo is too big for a 400....650 Holley is fine on my 400...

using too big a carb is just wasteful...if you cant use all that gas, its going out the exhausts...the best is a carb that has the right air mixture...the leaner , the better mileage , but also leaner runs faster...too lean you will run hot though

on a 5L (Ford ) engine I had in a fiberglass car, I started out with a 650....car got 14-15 mpg and ws running ok...installed a smaller carb , car was faster and got 19 mpg...bigger isnt always better