Hi Stopngo, Exactly what carbs did you get? Amal premier?
The sellers often sell a "universal" slide/jetted carb. They generally list the slide/jets in the spec sheet on their web page. So carb may or may not be best for your needs as sold.
On paper this should be simple but often it's not. Variations in fuel can have dramatic effects. I don't feel you are overthinking this. Worn slides will make bike do odd things just a trace off idle & rolling on throttle on slow curves & the like. Can cause spontaneous stalling at lower speeds also. Also will need to have idle higher than normal to keep motor from stalling.
The new Amal castings compared to 1969 & 1973 castings are different in many ways even though at a glance the same. The floor drillings for idle/transfer holes are different sizes. Not sure what you mean by larger diameter of where needle jet sits matters. I find the mixture between the old carbs & new premier to be same with same size needle jets. Changing needle jets gives the expected changes. I find changing slides & main jets give the expected changes as well. Read the notes below about air filters though.
The old needles often wear a flat spot and can score on the side that contacts needle jet. Also the needle jet wears oval from the needle contact. This wear especially on needle gives a rich spot in mixture. Generally around 1/8 throttle where we mostly ride. Needles are only 1 size, but there can be .0005" variations in straight portion diameter. .0005 will effect mixture as the clearances are so small.
I find with premiers they tend to come with a larger (removable) idle jet. This is correct & needed. The changes in the new design seems to require that.
Float level is very different on new carb as well. It will be higher than float bowl gasket surface & parallel with the surface. Sounds way too high, but is correct. Only an actual test of fuel level will confirm you true level, but I've found 3 recent premiers this was correct.
If I was doing the carbs, I'd leave new .106 needle jet in place. I'd put clip on bottom groove of needle. I'd leave #3 slides. I'd install 200 main jet from old carb. Mixture screw base setting 1.5 turns out.
This should allow easy starting & decent running for road test to warm motor & begin tuning.
I find for 10% ethanol fuel in California 5% richer across the board for mixture seems desirable.
Your exhaust & air filters make a large difference. I find the original type air filters with "fabric & wire gauze" made in UK work best as they are exactly like originals. I find the paper type Emgo tends to fit loose in housing & even with spacers made to fill the gap, then can modify the mixture. This upsets the entire tuning. I found it impossible to get a prefect tune with these. Ordering the original type, the tuning now made sense & changes to jets/slides gave the expected results.
Before carb tuning valve adjustment, timing must be correct. This is a given. Compression must be good meaning no leaking valves or horrible rings. As long as valve sealing is good, some ring wear & oil consumption doesn't seem to make a huge difference. Any valve leakage will make tuning impossible.
Tuning is a time consuming process. Plug readings can vary greatly depending on fuel. California fuel will look pretty much the same unless really rich or really lean. The ivory for lean, black for rich. A good mixture general looks a little rich with my fuel.
For tuning start with a road test say 65 mph up a moderate hill (if possible the hill). Motor fully warmed. Roll on full throttle & hold it. If motor misfires like ah ah ah, then runs normal backing off ah ah ah again rolling full throttle back on the main jet is too large. Reduce 1 size & try again. I always start rich & work down after 8 stroking. I leave the size where 8 stroking is 100% gone. Lean is harder to tell. Motor will gain very slight power when backing off throttle very slightly. Again I struggle with lean even after many attempts to feel this.
Rich & lean at lower speeds like 25 mph are a little harder & often takes some trial & error. Lean is harder to tell. Rich though backing off throttle to zero & coasting down in 2nd or 3rd if jerky & blah blah blah is probably too rich. Needle jet too large.
Remember needle is straight for a long ways so clip position doesn't matter until you get to taper. Many don't seem to understand this. Before the taper slide is important. Then taper/slide work together for a ways, finally 100% main jet.
Here's a link to tuning guide. Mark your grip! That's the only way to know what part of carb is in operation. The only way!
The link looks wrong, but is correct. Print the tuning pages & take with you on road tests. You'll soon see how it all works.
Take your time & good luck.