How old is the engine and have you changed the timing chain? It'll develop quite a lot of slack and the cam will retard many degrees. Not sure it would cause the stumble by itself, but if the chain is old and stretched, the engine will run sluggishly. With a new chain, there will be significant off-idle and midrange driveability improvement.

And as mentioned, timing marks can move; if you can check the mark vs real #1 TDC, that would be very good to do and know the results. Reminds me: I need to check mine!

The engine is a 1966 and is a 170 according to the fender tag it should be a 225. So someone changed it without knowing the differences, I bet they noticed when they drove it though. I had the timing cover off to change the gasket and seal. The chain didn't have much slack at all, it looked fairly new. But I got the car about 4 years ago. It had been sitting in a shop for at least 2 years before that not being started. So there's no telling how long it actually sat. I know when I took the valve cover off to check the valve clearance it was pretty dark and gunky. If I had to guess it probably has 20,000-40,000 miles on it. The gaskets had all been changed but over tightened or shrunk, because I had to change them all. Other than the slight exhaust leak from that pipe, it's a quiet motor, no unusual noises, and idles smooth, no misses. Well here's a video of the short test drive I did around the backyard. I'm still not confident in it enough to take it out too far on the road. This day in the video it was in the 40s outside and after letting it idle for over 5 mins it would consistenly cut out while giving it full throttle. I noticed the heat rise flapper was tight too, but wasn't sure which way was open or closed since I couldn't see the flapper. At the time of the video it had been running probably 20-25mins and it didn't seem to cut out anymore as you can see the full throttle run, that was gas pedal all the way down to the floor. We tried to reset the timing with it in drive to bring up the vacuum, but couldn't even get it past about 14, back in park and it was good 16-18. Fuel pressure consistent 5-6psi. Does it still seem like a float issue? I guess I can take the bowl off and look now. Everything else seems to be covered. Here's the link to my video, my friend thought it was a Belvedere but it's definitely a Valiant. https://www.youtube.com/watch?v=_27pC7XpBm0

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1965 Plymouth Valiant 200 4DR 2.8L 170CID
1973 Dodge Charger 6.6L 400CID
1986 Plymouth Reliant SE Wagon 2.2L 135CID