The 55 MPH speed limit did not come in until 1974, well after the last 122s were sold in the US (68). Before that, it was up to each individual state. Many of them set it at 70, and a few were higher than that.

I think the US market automatic 122s had a 4.10 axle.

The US Spec 1800ES automatic had a 3.91 axle.

All US spec cars had twin carbs, usually SUs, though some 140s had Zenith Strombergs.

An important thing to remember is that it is difficult to convert an automatic 122 to manual. The tunnel is much wider, and as a result, there is nowhere to bolt up the crossmember that supports the back of a manual transmission. It is necessary to fabricate a wider one that lines up with the holes in the floor pan and the rear transmission mount. By no means impossible, but far more involved than a simple bolt-in.