15 degrees is pretty cold! If you leave the choke on a little does it drive differently?
I doubt the curve is the real problem. Is it pinging badly? What is the initial advance and what is the total advance.
If total mechanical advance is limited to not more that 30-32 you should be fine. To much advance generally slows down our race cars at the the high rpm end of the spectrum, not the low end.

I would definitely check for air leaks at the intake and carb. Does changing the Accelerator pump shot help the bog/hesitation?

The electronic ignition I put in has an advanced street curve (Rick’s Mopars on EBay)

I will heartily disagree with Dan on the intake issue. That 2bbl Aussiespeed long runner is superb at low-mid range. It should be perfect on your truck if the carb is jetted right. Better than a 1bbl or BBD on a stock 2bbl intake. Yes, I have personal experience with that setup. I did not have fuel puddling or bad behavior at low temperatures, at least down to about 30 F. I did run the 500 2bbl, which works well on about any Slant (yes, I have run it on stockers and 11:1 race motors), but the 350 should be as good or better given the mild nature of your engine. That intake actually does NOT work well at high RPM and really shines from idle to about 4500 or 5000.

It would really help you to have higher compression and more head flow for your application, but it sounds like that will have to wait. I second Greg's comment about going to a 3.91 rear gear with those tall tires.

Lou