My personal opinion would be that the Paternal Arms would be the senior and, as such, would take positions 1 and 4 with quartered arms. As long as you had the necessary proofs of direct paternal ancestry, that is all that is important in Heraldry as the arms go with the surname of the original armiger. As far as I'm aware, you can call yourself anything you like, so you can always revert to the original surname for heraldic purposes, especially if you intend to approach the Spanish heraldic authorities.
If you are desirous of this outcome, may I suggest that you make contact with the Spanish Heraldic Authority through Jose Casas y Sanchez [ http://freepages.misc.rootsweb.com/~amateurheralds/membershi... ], one of the Vice Presidents of the Association of Amateur Heralds, who is a direct conduit to the Cronistas.