I have acquired a deck stepped mast for my Summer Wine which is currently hog stepped with a mast ram. I understand the reason for the puller, however would the ram not do the same job?

Yes it would, as long as it is rigged to do so.  Most struts attach to a lever under the foredeck with string to apply pressure to the front of the mast to resist the load from the gooseneck pushing forward under kicker tension which is otherwise resisted by lower shrouds.  If you also have a pulley system to resist the load of the spinny pole pushing backwards by pulling the strut forwards that will do the job of the puller.  However, deck stepped masts are usually MUCH softer and more bendy than the old Superspar M3's & Proctor D's used in hog-stepped rigs.  Sideways bend then becomes a problem and lower shrouds prevent this, struts and rams do not.  Also it is not easy to design a system that is easy to adjust but also when cleated resists moving in either direction.  Conversion to a full deck stepped rig involves quite a bit of serious engineering and if it is not done properly you will be very much worse off than where you started, with a system that doesn't work, breaks or both.  Have fun - after all, there are no restrictions on how you achieve things & there have been many innovations.

Thanks Andrew. I am having the king post fitted by a boat builder & I will be using lowers through the side decks to a purchase system. I will remove the ram and associated paraphernalia and use a thru-deck fitting ehaere the rem goes through the deck to run the puller through. having looked at some of the pictures it looks like people use a 6:1 system under the foredeck led back.