Do you have any space between the dishwasher and the unit it sits in either side?
Have you measured the gap the dishwasher sits in rather then the dishwasher itself?
Measure the gap from side to side which the dishwasher sits in and see how wide it is and let me know.
This is the bad news in im afraid. Older dishwasher were different sizes compared to modern buit ones. As they are now all around 600mm wide as thats the standered gap now so finding one that bit smaller there isnt im afraid.
Ive seen this problem a lot were you have older built kitchen units for older appliances and when you come to replace them they wont fit it.
The only other thing i can think off is getting a slim line dishwasher which is 450mm wide. The wooden door will still fit on on but you will have bigger gaps either side.
Ive had to fit them in the past for problems like this were you couldnt find that extra few mm to get a full size one. Then just fill the sides with a wooden strips down the sides to cover the gap.
Thats the only other option you have here as they only do 2 sizes now either 600mm or 450mm, Sorry.
Ok strange as height shouldnt be an issue with it.
Is your kicthen work top low down then?
Do you have a washing machine under the units in the kitchen?
Also did you measure the gap from the floor to the work top as 750mm?