Seems like you would want to check and confirm your tune up spec's before you start changing parts.
Initial timing?
Mechanical timing, is it working, how much total?
Vacuum timing, is it working, how much total?
Compression test?
Take a plug reading, are they all uniform?
How much vacuum at idle?
What is the float level?
Has the carb been adjusted, do you have any vacuum leaks?
When you adjusted the carb, are both mixture screws out the same number of turns?
Have you checked for fuel pump flow (volume) and pressure?
After you have the basis's in adjustment, you can easily determine if it's actually very lean by just holding the choke plate partially closed with your thumb at 2500 rpm. If it likes "less air", it will respond well to "more fuel". You can adjust both the idle and main circuits. Most were very lean from the factory by 1970 when new to meet emission standards. It worked better at higher altitude, but lean caused surging at low altitudes.
You need to figure out where it is lean and fatten it up in that area.
Drilling jets is a last resort, IMO. If you cannot find the desired sizes, at least use number drills to make your fine tune changes. Always a good practice to file off the jet number if you drill it to save the next guy working your engine. I was always told that the number on the jet was rated by fuel flow, not necessarily just the hole diameter. Jets with the same diameter hole and different jet numbers have a different flow. If you end up drilling, go in very small increments. The idle/low speed fuel mixture is in the emulsion tubes in the end of the booster.
I believe you will get a better result by diagnosing the problem, then focusing on that area. Are you even sure the jets are a original size (did someone already drill them?) after 45 to 50 years?
John