Too much Vacuum Advance

[69GMCLonghorn]

I recently bought another project. It's a bare bones '70 Chevy 1/2 ton with beyond lethargic 307/Turbo 350. I've driven 6 bangers with more power than this thing has. Anyway, I checked the base timing and it was at around 8° BTC, so I figured that was at least in the ballpark. Then I checked it without the vacuum advance hooked up and it was somewhere AFTER TDC. It has the stock 2 BBL Rochester on it. It seems to be getting full vacuum at both ports on the carb at idle. Currently I have it set at 8° BTC with the vacuum advance disconnected, which equates to about 18° with it hooked up.

The other side effect is when I fire it up when it's cold and the choke comes on, the vacuum drops and retards the timing and it stumbles until the choke warms up and turns off.

Is this a carb that about ready to crap on me? Or is there some other reason for this?