Well, I can't take full credit for this idea completely, it and similar concepts have been done for 20-30 years already in different motorsports of racing. If you have ever put a baseplate from an 850 onto the main body of a 750 Holley, then you have already inadvertently performed a similar modification. This was a popular mod 25 years ago or longer that improved performance. In a similar way, in racing classes where you are restricted to a baseplate of a certain butterfly size, usually a 750, then it was common to bolt an 850 main body onto the 750 baseplate to gain an extra edge, and it also was worth some HP to do this. We had a local circle track that required this for a class at one time that we did engines for, and I modded a bunch of these. In the same way, you can create a spreadbore type carb with the standard Holley bolt pattern baseplate by changing the primaries to ones that are smaller just like the ones in a 390 or spreadbore carb. There are several ways you can do this, it's not hard to figure out. But you basically end up doing the same thing you are now, reducing the effective size of the primaries, except now you would be doing it in a way Holley already has done it. You won't have to plug passages here and there and block off one hole barrel completely. And you still have the benefit of a larger secondary with the butterfly size of a 600 like you have now. And it will be as easy to tune as any other Holley, just simple jetting and squirter selection, idle mixtures, etc....
