I had a flash of insight and decided to measure the hole sizes on a 044 carb. That carb is generally recognised as a good upgrade on some saws in the 70cc range.
First thing I learned was my assumption that the second air bleed hole would be the biggest was wrong. On both the HD5 and the 044 carb, it was the first air bleed hole that was the largest.
I measured the drills that would fit through the holes, the holes were probably just a tad larger in order for the drills to slip through the holes.
On the HD5 the first hole used a drill .62mm and the second hole used a drill .48mm.
On the 044 carb the first hole used a drill .72mm and the second hole used a drill .62mm.
For a 77cc engine it looks like the Walbro could be drilled for the 044 specs and be close. I'd probably go a size larger on the first hole.
The optimum holes sizes can only be determined by trial and error. Ya gotta go to far to know where 'to far' is. Somebody that has a few extra Walbro carbs sitting around could try a few combinations and come up with something, but I don't have that option.
Good luck.