No, you can not assume all 44 pin quad flatpack are the same. Many probably are, but the one you rely on may not be. Surely the mechanical information is available somehow. You didn't link to the datasheet, so I can't say for sure how to chase this down. Good datasheets contain the mechanical information. Some companies (Fairchild comes to mind) use separate datasheets for each package, and refer to the specific package info in the part datasheet. It's a bit of a hassle to have to find and download the mechanical datasheet separately, but the information is still all there. At worst, they may reference a JEDEC registered package. That's more of a hassle to dig up and find, but it's still available.
If the datasheet really doesn't include or reference the mechanical specs somehow (I seriously doubt that), then that should be a warning not to use parts from that company. You don't say what this WDCxxxx part is and what it does, but surely there is another way to solve your problem by using some alternate part.
As a last resort, contact the manufacturer and request the mechanical information. But, any company that doesn't make it accessible in the first place probably won't make it easy to talk to someone intelligent either, and then you have to wonder about accuracy of any information they may tell you over the phone. Unless they can send you a datasheet for the super secret mechanical info, I probably wouldn't believe what they say anyway. I'd probably use a different part from a different manufacturer before wasting time trying to talk to someone to help me at the first company.