72 chevy 350 running rough under load!

[geezer#99]

You're not an expert Donut but you're thinking like me. I suspected the normal lock up o/d no rear gear scenario too. Good at cruise but till you get there a real dog. My 84 with turbo 350C and 3.42 shifts into lockup real quick and runs the same till I get up past 30mph. I shift mine manually. Put it in second at the lights and shift to 3rd when I get up to 25-30mph.
As for shifting manually it won't hurt it. Some people hook a switch into the lockup circuit to delay it but usually forget to flip the switch.
FWIW your hydraulic lock up if it's working full time will drop your idle rpm from 750 rpm right to zero when you put it in gear.
Don't worry about seeing over 50 degrees at times. Some factory set ups had close to 55 degrees total.
Your dyno guy or you or anybody can't set your a/f ratio with the mixture screws. You do that with jetting and/or metering changes. Your mixture screws are for idle mixture.
The other major problem you got right now is your local weather. Waste of time tuning until it gets back above 15*C.
If your motor tolerates 12*, starts and runs and doesn't ping then try shifting manually until spring. Right now you're banging your head on the wall.
And my apoligies for coming across as such a hard....s!