Howdy SkyTrainBoy,
To begin with, let me wish you a big hearty "Welcome!" to the site.

Please hang around awhile and pay attention to the 'bunchO'bloke's' we have around here......they'll never lead ya wrong.
Now, as to the .50 cal on a Cromwell.............................................................................................afraid not.
As MUCH of an armour variant lover as I am, I cannot come up with a version I'm aware of that had an American .50 mounted on the turret. Now, that doesn't mean you can't mount the bloody thing...
it just means you've shifted into the 'what-if' catagory, that's all
Regards,
Tread.
BTW.....I would dearly love to see a pic of one.