I'm putting together the kit I need to do a ECM swap on my '99 GMT400 truck.

Is there any pressing reason why I couldn't use a P59 1mb ECM from a van with IAC control in lieu of the 512kb P01 0411 that everybody seems to use for that swap? I searched on here and found a low res L31 tune from a P59 equipped Mexican GMT800 truck, so I'm at least broadly aware that it should be possible. Has anybody done it? Is there some good reason not to?