Recommended by what Manufacturer? The engine turns over fine and starts within seconds when the battery charge is fresh.
I wish I could find a Long and Wide but Short battery...they all get Taller as power increases. I gotta keep the height down...
I did a search for a battery for a 1963 Buick 215 engine and thats the results I got.
Group 24, 34 Battery with 700-750 CCA. RC 120-140.
I can see the engine starting with no issues with a fresh charge but the reserve capacity of the battery your using is very low and probably will take some time to fully charge.
Wiring 2 low profile batteries in parallel would increase the reserve capacity and keep the voltage the same if that would help.
Have you checked the voltage output of the alternator at idle?? with a load applied to the system??
I use to live in Wisconsin as a kid and always looked forward to the first snow of the season.