Narrow Body cars are exactly that - the tub is actually narrower. Before Morgan moved to a common chassis there were actually two widths available depending on model... mainly down to the introduction of the +8... though there are a few narrow body +8's built very early on.
Lowline - high and low cowl models
Longdoor - there were models built after a certain date where the doors are longer.. the earlier cars were short door and could pose problems for the taller or larger driver.
What confuses things is that whilst most newer +4's had more grunt and were wide body the narrow body was felt to handle better (for whatever reason) however the 4/4's always ended up with less power. This was resolved with the limited production 4/4 75th Anniversary model which ran a 2000cc engine despite being a 4/4. Therefore +4 Grunt in a narrow body chassis... last seen in the very early +4's (think TR era).
There is more detail to read up on in the Morgan history but this is why the now fabled Narrow Body 2000cc cars (as in the 75th Anniversary Model) are much sort after.
I have to say that the two 4/4's I have both running Zetec E engines. One 73 running a Black-top Zetec E with 175bhp and an 85 running a Black-top Zetec E with 250bhp would embarass most.. and at sprints can be seen giving the mid level Caterhams and Elises a run for their money. They are both much more nimble than the Class B 3.9 +8 I had a go in.
The Morgan suspension seems to be much more at home on the track maybe than out on the bumpy poor surfaced roads... where-as on a track the issues around progression and suppleness are seemingly less of a problem and many racers going for harder springs and less damping.
One other note that is worth adding which nobody seems to think about is that Morgan generally use off the shelf gearboxes.... the clusters within them designed for heavy saloons or rev ranges that don't match the driving nature of the Morgan. Quite often all the adjustment is done on the final drive in the rear diff. We have seen on the 4/4 the availability of a 4.1:1 diff as an option.
By keeping the same engine and swapping the cogs in the gearbox you can find that a smaller engined 4/4 becomes a very potent weapon in comparison to the larger cars.. with less weight it can often be a little more cheaky round the bends.
Looking at the level of development of certainly the more powerfull 4/4 I have I reflect on it as getting to the point of bolting Formula 3 grade components to a piece of wet spaghetti with the aerodynamics of a brick.... even with the double wishbone set-up that is available... look what you are bolting it to!! The top end Class B and A cars which run a full cage use the full allowance of mounting points to effectively build a new chassis (as in the roll cage) then bolt the car (the more twisty part of the pairing) to it. Cages starting from the rear suspension mounts at the back all the way forward to brace the front cross-head.
The number of times I've mused over what I would do regards a whole new chassis, suspension and streamlined body.. all be it in the Morgan theme...........................................hhhhmmm