First thing is to figure out where the restriction to the top end is going to be and then go from there.

The hole in the bushing can be the restriction but I wouldn't recommend it since it can be easily plugged up with debris and there is no easy way to do a clean out.

I think the best system is to use the clearance in the lifter bore as the restriction. But for that trick to work you have to use the correct lifters.

The location of the hole can also be important depending on the lifters.