There aren't any custom-fit covers currently available for a 1957 Morris Minor Van so you'd be stuck with a generic cover like the Adco SFS AquaShed Cover for SUVs # 290-12285, which is designed for SUV's up to 196" in length. This would most likely leave quite a bit of slack so you'd have to get creative with the included straps and possibly some extra bungee cords to properly secure it.
I do know that ADCO will custom make covers, so if a universal-fit cover isn't what you're looking... view full answer...