1974 Valiant Scamp, timing retarding when revved

Way-to-go, ET
See you're learning already.....The engine is telling you it wants more initial, and you are hearing it.
Forget the specs. They were established to limit certain pollutants, and do nothing but make your engine sluggish. When they were brand new it was tollerable, now decades later, not so much.
But if you advance it more, the idle will go up, and at 850rpm already,you don't want this. So your reflex will be to slow it down with the curb idle screw; DON'T DO IT! That will mess up the T-port sync! If you have to slow it down, your only option left is to reduce the air intake somewhere else.
To reduce air intake somewhere else,your options are:
1)Making sure the secondary plates are fully closed, but not sticking; BOTH of them EQUALLY. It can happen that the secondary plates were delivered to you with the plates not synced. On a big cam engine this is mostly no big deal.But on a stocker this is a very big deal. If one plate closes and the other hangs open a bit, then two things happen; A) the idle speed may be higher than desired on account of the extra air, but worse is B) this lopsided air delivery has no fuel in it. This causes a bit of an idle-tuning issue on account of the mixture screws have to also be lopsided to compensate. But it it's not even that simple;the two back cylinders on that plate are gonna get the majority of the dry air, and will run lean. While the two closer to the front plate will be a lil fat. There is no tuning this out. So if you find this you gotta fix it.This is mostly an idle issue, but can also affect the low-speed tip-in.
2) the PCV can be swapped out for one with a lesser flow rating. Of course you are gonna have to buy about ten different ones to maybe find one that sorta works. Or you can take yours apart and modify it. But before you get tangled up in that,simply clamp that PCV line off and see how much change it will actually make and if the headache will actually bring a good result.
3) while you are clamping things, you might as well clamp everything in sight to make sure no vacuum devices are faulty, especially a brake booster
4) the intake gasket needs to be proven to be NOT sucking air, as well as the carb base gasket, and the throttle shafts. This last one cannot be sealed up 100% tight, and some leakage here is inevitable, so don't pull your hair out trying to figure out how to overcome it. As long as the primary shaft is not flopping around and causing problems with the T-port sync, forget about it.

So if you can't find any air leaks, and the idle gets higher than 850 in N/P, while dialing in a few more degrees, then I really cannot see a good way to increase the idle-timing beyond the 10* that you currently you have.
Now having advanced the idle-timing, this has also advanced the power-timing. This is something you have to prove acceptable, or change it to be so. Max power-timing on a stock,open-chamber headed low-compression teener is probably 36*. Some won't even take that much. And I guarantee you won't feel the difference of 2 to 4 degrees, on the street. So play it safe and modify your dizzy as may be required to limit that power-timing to say 34* plus/minus 2*.
AFTER that is done; you can experiment with weights and springs to bring the mechanical advance in as fast as she will take it without detonating.
AFTER that is done; you can start experimenting with the Vcan.

But right now, limiting the power-timing is job#1.