I am looking for a head to use on a point of view dolly, it does not have a ball so I need to use a flat connecting head, I am not sure if the heads with a ball have that ability if I take off the ball base on the head? is that possible? I have a HDV 501 that has a flat connector I could use, but I prefer the HDV 503 which has a 75mm ball and is attached to a better tripod, so I was thinking of getting another but can I use it on a flat surface if I remove the ball socket?

yes you can. The ball head is nothing more than a connection between the head and sticks.
To remove the ball from the head simply unscrew the 'handle' of the ball to remove the head and ball from the sticks. Then on the under side of the ball plate there are or should be 3 tightening screws. Use a small screwdriver to loosen those then simply give the head a good twist and it will screw right off. Weh nyou place it on the other mounting plate make sure to tighten up the screws underneath to lock the head in place so it doesn't twist around.
Don

Thank you I will try that