Plastics can be stretched out similar to ski boots. Be careful about voiding the warranty on them though. Very rarely when I worked in a shop and fitted boots did I find some, especially plastics, that fit a person perfectly. Buy the boot that fits you the best (not perfect) and start customizing the foot beds. If you have a problem with your heels slipping (in a boot that accomodates your forefoot) there are a couple of lacing tricks and/or get heel inserts. That's a lot easier than stretching the front of the boot.