Frankly, I'd prefer the Nissan 1200. It's a much more modern basic design and has an alloy head, so it's lighter and I'm sure it's oversquare.
The 1275S engines in my racing days were stretching to get 130hp, the Nissans with similar mods (like the Corollas - another option) would get 145hp. I would like to go with the SU carbies, however. They're nice, need no tuning once set up and give good power and excellent economy.
With the Nissan you might also find a 5-speed gearbox. They came in some models. Another possibility, though rare, was what was called a GX head, which had better porting.
BMC rear ends had 3.78, 3.90 (very rare competition model), 4.22, 4.55, 4.875 and 5.125:1 ratios. There is a similar spread of ratios among the Nissans, but harder to find the lower ones (which I'm sure you don't want).
The narrowest Nissan rear would be from the Datsun 1000 of the mid-sixties, I'd think. Again, Corolla would be an option. These are strong and light, though maybe a tad heavier than BMC axles. Most certainly less prone to troubles, however.
If you do use a BMC rear, get sealed bearings for the semi-floating setup, DO NOT use the tab washer under the big nut (they hammer themselves thin over time and let the nut come loose), but use Loctite instead. It would be a course of wisdom to polish the radius inside the axle flange as that's a point of failure, and if you have access to a machine shop it might be good to neck down the axle to the inner diameter of the spline to give it some compliance so it won't break so readily.
The best insurance against breaking axles, however, is to have the rear suspension set up so it neither lifts rear wheels or lets them bounce too much.
All of that said, either would be satisfactory for your state purposes. 65mph would be well within the bounds for cruising, though wheel size and gearing have to be considered. That fifth gear in the Nissan box would be a definite plus here, though they do adapt easily to the Morris engine.
