Snailmog,
In principle fitting a higher ratio crown wheel and pinion will give you the more relaxed motorway cruise you are seeking. With your fourth gear ratio you'll need about 3.055:1 for 3,000rpm at 70mph. However, if you stick to the four speed gearbox and change the back axle ratio, you need to remember that you will also raise all the other ratios as well. In particular, your first gear ratio will increase, possibly to the point that setting off on a steep incline may need excessive slipping of the clutch. Additionally, acceleration in second and third gear will be dulled and you may find motorway inclines will be a struggle in forth.
In the 1970s and 80s, when the Ford Kent cross flow engine was in everyday used, 3,500 to 3,750rpm at 70mph in top was the norm, so I suspect your 4,000rpm at 70mph was chosen by Morgan to give spritely in gear performance.
So, looking at DaveW's post and IMHO, I would think that keeping the back axle ratio standard and trying to source and fit a five speed box which keeps the same first 3 gear ratios, a 1:1 fourth, and with a 0.87:1 overdrive fifth would be a better solution.
