Your 4777-7 has throttle bores of 1.688" or 1-11/16" or 42.9mm.
Increasing the hole sizes in your manifold will not hurt, but I don't think you'll see any significant HP increase. The stock manifold is just not a high performance piece that would respond to such efforts. I really think you'll be happy with it as-is for now.
I would suggest a thermal spacer. I looked at Summit Racing and four hole Holley thermal (phenolic) spacers have 1.563" or 1-9/16" or 39.7mm bores.
So, if you ultimately decide to enlarge the manifold bores, you should bring the spacer to your machinist when he does the work so he can make the holes in the spacer 43mm too.
And make sure the machinist plugs the holes at the ends of the groove too.
Good luck