Help! I need a good mechanic!

Well, I see that almost everyone is going after the fuel system, and probably rightly so. But I have some other ideas. Longshots to be sure. But here's my thinking; There is enough fuel in the float bowl to make a quick run to well past 20mph, even if no more fuel entered the bowl, once you started the run.
So with that in mind, the plugged pipe is a good possibility. Except for the history, which said it ran well, after the carb job, until about late last summer or fall.A good second possibility is the fuel,except again it ran good until about last fall.
What you didn't say is that it stalls.And you didn't mention any bucking or farting, which is a sure sign of running out of fuel. And you said you couldn't get but 20 mph out of her.
But you did say it starts to bog out.And timing changes made it worse.And it's a 66 slanty, (with points Ima guessing).
So I'll throw my hat in with 318willrun, as to the choke somehow engaging. This will of course drown the engine, neatly hitting all the above bases.But it usually stalls out.
And then,of course there is always; tank venting.Or rather lack of venting.
But if it's not choke or vent, I can easily see a faulty fuel delivery issue, but this is also easily tested with a fuel delivery test, right from the fuel pump, before ripping it all out.
And yes today's oxygenated fuels do have a very short shelf-life. However, you also said it starts and idles fine.Stale fuel will not behave this way.
I have seen a similar thing happening with a distributor with so much bushing play in it, that when the vacuum advance engaged, it closed up the points.
So, a quick check of the sparkplugs will prove the choke theory, and the lack of fuel theory, and disconnecting the Vcan will neatly rule out the timing issue.

Now, here is a really longshot; If the camchain jumped just one tooth, and someone reset the timing, it might act like this too.The plugs will be clean, and disconnecting the Vcan will make no difference. Of course an entire new fuel system won't cure it either.
So, if the above quick checks come up OK, and, while it is a really long shot, I would next go with a compression test.Really low numbers could point to the chain.Of course being who I am, I would check and adjust the valvelash right away too, cuz while I'm in there I can check where split overlap is in relation to TDC-Exhaust, and if it's off by more than a few degrees, I'm gonna pull the chain cover,and have a look-see.As far as theories go, this one sucks tho, cuz the carb-job would have had no positive affect. So I would leave it to dead last.

FWIW; if you had mentioned bucking and farting, but not stalling; I would have gone straight to the power circuit in the carb.And if you had mentioned stalling as you took your foot off the gas, I would have been looking for water in the gas, sitting in the bottom of the bowl.And if it is water, it will still be there on Monday mourning...........But you didn't mention any bucking, farting, or stalling......................