I would try it out with no power valve. Take an old one and remove the rubber, then melt solder into it. I've done them both way. You may need to put bigger jets in.

You block the power valve while checking the main jet and NOT UNDER HEAVY POWER, you can fry motor doing it. The valve should not be opening when main system (boosters) start up, rather, somewhat after that. If you are getting the flat spot while driving around like normal cruise then problem is NOT the valve. Could be trash in a main well from the drilling or the brass insert in too far to make a restriction. Going to the full size restriction of a dead stock carb should not work either, they restrict them for a reason, it being too big for a four cylinder. They are usually up around .040"-.055", these will like around .020"+.

You end up with two main jet choices with a blocked PV, one is good driveability at cruise and low demand throttle and the other is high demand full throttle power. There will be a separate main jet that motor likes for both, then you figure the difference in cross section of the holes and that ends up being the power valve restriction size. The power valve supplies the difference in fuel between low demand and high demand requirements. The jet size the motor likes for full power will be too big for creeping around in slow driving and the motor will be too rich then. Why the system was created to begin with.

Think kickdown on an automatic transmission, power valve comes on about then, the demand for extra load. Maybe a wee bit sooner but not until then, it needs to be closed for clean normal light load driving. Why blocking it to test drive at low load is useful.

If the idle is still set to make the throttle plates go higher in the transfer slots it never will run right, the transfers should be barely showing like maybe .020" of the slot and no more when carb is pulled to look at that after normal idling speed has been set. Too high in slot makes for too rich an idle, you cut back on the screws to cure that but then when you increase throttle no fuel left for transition load because the high slot condition has used it already. So, too rich, then instantly too lean right around normal cruising spot. The entire lower rpm/load workings of the carb are set up based on that idle transfer slot relationship and many mess up there.

If an old carb can always have some sort of blockage in a metering block well. A few of the carbs they put idle feed restrictions on the tip of a long tube that presses into the well, if those block up the tip almost impossible to clean.

that method of figuring out the power valve restriction size makes sense.  as for the idle transfer slot setting, i'll check it again, but ive tried it from as low of an idle setting to stay running all the way up to too fast to comfortably stop the car while in gear.  no change in flat spot.  this is without adjusting the idle mixtures screws.  i tried adjusting them as well without changing the idle speed screw.  I tend to try and leave the idle mix screws at or close to the baseline 1 1/2 turns out.  guess i should try setting them with a vacuum gauge.

one of the racer walsh restrictors is pressed in farther than the other.  There could certainly be a blockage somehwere in the wells like you said.  Im afraid that may be the problem whether the drilling caused it or something else.  im going to see if i can remove the restrictors and find anything.