Steering effort depends a lot on your alignment too, changing the caster a few degrees can make a big difference in steering effort. The more positive you run the caster, the harder it will be to turn the wheels because you're basically lifting the front of the car more because of the added camber when turning. Tire width has an effect as well, so, the wider the tires the more effort.
The 20:1 boxes were not a factory option. They were a replacement gear offered by Mopar Performance, and through the kit car program then sold through the old DC/MP catalogs. Factory manual boxes are either 24:1 or 16:1. The factory power steering is 16:1 as well. The MP 20:1 worms were available for quite awhile, then fell out of production. I'm not sure if they're currently being produced, it seems like Firm Feel offers that ratio still so they must have a source.
I run a 16:1 flaming river manual steering box in my Duster. It's a 340 with iron heads, but I have an aluminum intake, water pump, radiator, headers, tubular suspension components, etc. I also have 275/35/18 front tires, and run +6.75* of caster. I've run the caster as high as +8* and as low as +6.25* with the 275's. As you can imagine +8* made parking lots pretty miserable. But I increased it back up from +6.25* to +6.75, I'm pretty happy with that. Below 10 mph it definitely takes some effort, and you want to plan everything so you're turning while you're rolling. "dry steering" while stopped is really difficult, but you shouldn't be doing that anyway. Above 10 mph it's great, the steering feel is great. Not super heavy, but very reactive. Probably not a set up you want on something you're going to parallel park all the time, but my Duster is my daily driver and the newest car I own at 1974.
