Well classicguy... me and the other 250ci folks knows A LOT about low HP ratings
... " days ago i made 450 kilometers (280 miles more or less)... My car has no mechanical upgrades what-so-ever and i never even do nothing to improve stock things... Even my shocks are factory so.. riding my stang is NOT as riding a "restored" car...
For that experience i can tell you that even the 250 ci is more than enough to the current traffic speeds... The problem is the 3 speed manual trans... It is ok to do city traffic because the car is still quite fast but in the highway, the trans limits the engine all the way... The comfortable speed for the car due to RPM is between 55 and 60 miles and that is it.... The car going at 55 is fantastic... at 60 he is asking for another gear...
Dont get me wrong... this is not even close to its top speed, even with my old and 40 year used configuration... if you press the gas pedal, it quickly goes from 60 to 80 mph and that was when i felt uncomfortable with my suspension and because of that, back to 60 and enjoying the ride
If you are not going to race the car, any engine will fit you, especially for highway rides... i´d worry a little more about the grarbox and rear axle to get a good ratings at the speed you want to drive in the highway...
For the record, my car at 60mph rated 7 km/L in fuel economy... something like 16,6 miles/gallon... i was expecting a little better than that... GRRR