Which SBM for the Mopar vehicle

StreetSleeper said
How much engine (torque & hp) would you want to have to safely drive in heavy traffic for:

Some of the B bodies (since size varies)

318

StreetSleeper said
C bodies

360

StreetSleeper said
FMJ bodies

No different than an A body- anything from a slant to a 360.

StreetSleeper said
Single cab truck -assuming potentially towing a vehicle once or twice

Or larger truck -(assuming potentially towing a vehicle once or twice)

318/360. Single cabs were available with slant and V 6s, so a smallblock is no issue, even with "towing once or twice".

StreetSleeper said
Would you ever tow with a car?

Hell yes. I pull a 16' camper, a 14' boat, and a 10' utility trailer with my '66 Dart wagon. I wouldn't hesitate one minute, as long as the vehicle is set up properly to tow. I've got a custom receiver hitch, load equalizers and an electric brake controller- all common stuff you'd need for towing with any car.

StreetSleeper said
Where I live there are SHORT ramps into a busy express way with semi’s driving 70-90 as you merge into traffic. And freeway crossings where semi’s are driving 60+ while you have to gauge, and sprint in front of them to cross the road.

How much engine size or <hp+torque> would you choose for those vehicles with a small block, and when would you guys switch to a larger engine?

The stuff you're talking about here isn't so much a matter of engine size, you're better off worrying about rear end gearing. Bigger, heavier cars could navigate traffic just fine with a 273 if it's geared right and running good. Something around a 3.23/3.55 and a good running small block (even a slanty) of any size will be just fine for navigating daily traffic and accelerating up onramps and through intersections. Just remember that as your gears go up (numerically) you'll gain torque, but you'll decrease your mileage and increase RPMs at highway speeds.
I think you're overthinking this a bit, you'll be fine in any ABFMJ with any smallblock, in a C body or truck I would gravitate to a 360; a big block if doing frequent towing, but that's not what you asked.