You're not far from me at all. Let me know when there's a good cruise-in down your way and I'll come by and we'll hoist our favorite beverages and talk cars.
drill and tap passage "H" to 8-32. Buy some (preferably) brass set screws (McMaster-Carr) and some numbered drill bits, if you don't already have them. Drill an orfice in the set screw as required.
That's the how. Here's the "when should I".
As a last resort!
As I previously stated, this is a trick that's sometimes handy on cars with low idle vacuum and high cruise vacuum after you've reliably determined that the transition circuit is sucking too much fuel at cruise. If you've reached the end of the road with smaller IFRs, and you're satisfied that you've done everything possible to increase idle vacuum (which will allow you to further decrease IFR size) this might help.
I use an Excel spreadsheet to keep track of changes, and I let Excel do the following math. Measure the length and width of the primary t-slots, and get the area. Next, using pi r^2, figure out a hole size that has roughly 75% of that area. That's a good starting point. Do this math every time as a sanity check.
This is a far less than perfect solution, but for those of us working with basic tools in our basement, it's all we've got.
My personal experience says that t-slot restrictors will help in situations of low idle/high cruise vacuum, but not totally fix it. What happens is, the restrictors get small enough that you eventually create an off idle bog at the same 2 - 3% throttle position that's screwing you at cruise, and this bog shows up before the rich cruise problem is totally fixed. Remember my previous comment that orfices sized for 8" hg are too large at 20" hg.
I'll pass on your intake manifold question. Modern single plane designs have surprised me. Maybe you can find someone who has experience with both of the manifolds you're considering on the same engine.
And the big problem with spreadbore carbs (as I see it) is lack of manifold choice. Not much out there, comparatively speaking. But if your car is truly a street car, they deserve some consideration.
