It is possible to shorten the zip fronted type by about 4'' if I remember correctly because the zip and the side vents stop about 4'' short of the rest of the hem if that make sense.
I have a few different Swanndri items and they are excellent, very warm and silent, very hard wearing (make sure you get the correct size and style as you will have it for a while!). Its true that they are not waterproof and, if they do get soaked, they take an age to dry.
Best to buy from somewhere where you can fit them on as the sizes do come up big.
Woodlore (Ray Mears's Company) used to stock them and they are in Sussex so might be worth a look. They don't have a shop but are very helpful so might let you fit one on in the warehouse.
Glyn.