The perfect setup utilizing an M3 50 cal tripod would be to put the A.A. 50M1 elevator on it, since it has a large pintle socket on top that will hold the M142 mount without modifications and has three extra legs. That would be very stable, but not real cheap, since you need that and an M3. Probably looking 8-900 for the M3 and 500 for the AA mount.
I have a spare elevator mount for sale that does not have the .50 cal cradle, so it's ready to go should someone feel the need? I don't have any spare M3's, but have friends that may?
I also have a spare helo mount for sale, but then you need the cardboard boxes which are not that great and it's big. Best bet is to go with the old USGI heavy canvas one with supports or the Capco boxes. those both clip right on and are not soft and saggy.
Adding any extension to an M2 tripod will not work out well as MG08 points out.