Keeping up with traffic could be better served with a transmission and axle ratio combo that keeps the engine in an RPM range it's happy with, that is to say lowish. Flatheads were notorious for breaking cranks if you spin them up over farm tractor RPM for extended periods.

I'd look at a T5 or similar and a 3.23 or 3.55 depending on tire size.

If I was building the engine for long term reliability I'd build it like any other, a light piston and H-beam rod combo with the crank properly machined and balanced. If the crank needed to be turned down to make an off the shelf rod work, I'd offset turn it for some extra displacement.

Add dual carbs and exhaust to suit.

Kevin