As for the heat riser passages . . . in your climate, it might be advisable to leave it open.
When I got my rebuilt Chevy 350 put in my '77 Camaro, one of the key things on that engine were the '86 Corvette factory aluminum cylinder heads. Being that the '86 Corvettes were all Tuned Port Injection FI engines, no head riser ports at all. I didn't worry about that as I'd had some friends (down here in DFW) that regularly blocked the heat riser ports in their Chevy small blocks, as that was what "hot rodders" did. My intake was a Holley Z-LIne intake (single plane with a full plenum divider and the "resonating channel" between #7 and #8 cylinders). Carb is an OEM replacement 9895 Holley 4175.
The first time it finally it hit cold weather, it was obvious that it took a bit longer for the engine to get warmed up, er for the engine's cylinder head heat to migrate into the intake manifold. With EFI, all of the mixture stuff would have been computer controlled, but not with a carb. It indeed was a bit more cold-natured than the prior iron-head open heat riser engine had been. By the way, the heat riser was eliminated as the heads had no related passages.
One improved aspect of the alum heads was that the heater worked sooner! As the aluminum transferred heat better and wasn't quite so much of a heat sink as the iron heads were.
I generally like to have my carbs set up to be on base idle within about three blocks of driving from my driveway and initial cold starts. NGK V-power plugs helped that, or regular plugs modified in that orientation. NGK Iridiums are now in it. Once the engine fires, oil pressure comes up, the car moves. A quick pat of the accel pedal for a bit of pump shot will keep it running, if it appears to falter. NO letting it run in the driveway to warm up, unless the glass is iced-up AND the car is locked.
Therefore, in the extended cold season y'all have up there, compared to DFW, I'd recommend you keep the heat riser passages open rather than blocked. You might still remove the valve or tack it open, or even get a good rebuild kit and THEN ream the bushings for the shaft so the shaft moves freely in them. On our '66 383, the valve would usually hang 1/2 open, which tended to cause no problems on the single-exhaust engine. On my '70 Monaco 383 4bl dual exhaust car, I tried to keep it working, too, for general principles. I knew what the valve was for and why it was there. The car ran good as it was. What really was great about that Code "N" 383 was how easily it sought to move the speedo needle toward "triple digits" with just a nudge past 1/2 throttle. Making those gloriously happy engine sounds as it did!
On the later EFI Chevy heads, it was mentioned that without the heat riser passages, the exhaust flow could be better planned for in the port configurations. Rather than the added turbulence from the heat riser passage configuration and flows. I suspect some of those things would still be at play with a cylinder head that was configured for heat riser passages even if the passages were blocked-off at the intake manifold gasket. The reason those passages existed in the first place was for cold-start drivability via better mixture distribution in that operational mode AND at cruise conditions. For a "full-race" motor, even one on the street, not having good cold-start drivability is not really as much of an issue as getting to the finish line first. Getting it decently warmed up enough to "run" is just part of the equation for that type of motor. NO need to put up with such things on a street motor with a milder cam, to me. Much less in 28 degree F ambient temp and 25mph wind!
To me, all things considered, I'd leave it open. Others might have different sensitivities to these things, which I understand.
Enjoy!
CBODY67
