I measured my rocker shafts OD at .8725" and the rocker ID at .8765". That gives me. 004" clearance on each rocker. That's much more than I was expecting. Restriction size will be a guessing game for sure.
I seen a thread where member Clanton, used a modified mig welding tip under the rocker shaft. I tried the fit, and it fits really good. The tips will need to be shortened (I think he mentioned 1/2") to fit in front of the rocker shaft stud, but after the stud is put in, the orfice tip cannot come out. This way I can make and try several different sizes, simply by removing the rockers. I'll start with. 060" as an initial size, and go from there. I need a set of Andy's "see through" valve covers! Lol. BTW, I'm using the Mopar 4006452-A aluminum heads.