I can only speak about the MT75 in my Zetec engined car but FWIW.......
There is no doubt that the ratios are not perfect for the Morgan. The 1st gear is way too deep and the step up to 2nd is therefore too wide. The Mazda gearbox has a closer ratio gear set and is far better suited to a light sports car. Having said that you only really notice it if you are doing a full on standing start or if you want to change down into 1st on a fast approach to a tight hairpin. Otherwise you don't really use 1st in normal driving.
Again, I can only speak for my gearbox but it is one of the sweetest changing gearboxes I have driven and it really only needs finger tip movements both up and down the box. My box has done 60,000 miles and I have heard that they can be a bit tight when new.
My personal opinion is that whilst the MT75 should not be a deal breaker on an otherwise perfect car, I would choose the Mazda box if it was the toss up between 2 cars and in the age range you are looking at you are getting close to the Mazda gearbox series cars.
I say this simply because I do like a spirited drive and I do notice the gap between 1st and 2nd gear ratios. Many others I've spoken to don't see that as a problem.
