This is common if the T-port sync is out. There is just not enough fuel through the idle circuit by itself; it needs augmentation from the transfers.
In truth, the transfers are your low speed circuit with augmentation by the mixture screws for idle and off-idle tip-in.
So IMO the answer to your problem will probably lie there, assuming; the compression is even,the valves are not set too tight,there are no vacuum leaks of any kind, and that the float level is properly set,that the gas is fresh, and that all your pollution control equipment is properly hooked up and functioning. That's a lot of ifs.
But when you are dealing with just 6 little pistons that are going on 50 years old,there is gonna be a few cobwebs to iron out.
If you want to, you can check the T-port sync first, but that may be misleading, this early in the game.
Ideally I would start with a compression test, followed by a valve adjustment and then a second compression test, to make sure I didn't make anything worse.You can tear your hair out chasing this problem for 10 days, when all along it had low compression, and needed the valves properly lashed.
I can give you 5 things to do, and when nothing works, you will think me a fool, when all along you had 1 or 2 cylinders with low compression.
Your slanty has this firing order 1-5-3-6-2-4.
It fires the first three on one revolution and the following three on the following revolution. That's right it fires only 3 cylinders per revolution. They are thus 120 degrees apart. So long as no two cylinders in a row in the firing order, are low in compression, everything seems fine. But if say 1 and 6 are low, or 5 and 2, or 3 and 4, then it will idle like a 4 cylinder, the remaining 4 taking up the slack cuz somebody merely cranked up the speed screw. But when you put it in gear, the engine just doesn't have enough power to both stay running, and drag those two lazy ones around too. If two cylinders in a row are low, pow! out go the lights.
Low compression is occasionally from a too-tight valve lash, but more often is a burned valve issue.
So that is where I would start;namely, the compression test.
.
