from my personal experience and from what i know..you have to do the 6 years...
When i was applying for the usmle, the school told me to apply as a graduate since i had finished internado and servicio social ...and was just waiting to take my examen profesional..well the ecfmg didnt accept my application and told me to reapply as a student because in their words :
Based on the information in the letter you submitted to ECFMG, you have not yet completed all requirements to obtain the Titulo de Medico Cirujano from the Universidad Autonoma de Guadalajara. These requirements include completion of the pre-graduate internship, the social service year and the Professional Examination. According to information in your application, it appears that you are presently a student officially enrolled in the pre-graduate internship or social service components of the medical school curriculum or for the Professional Examination.
That is straight from the caballos boca as they say.
Also, as for spain ...2 things one .. i know a lot of guys were going there to work or do residencies but they did it after servicio social.
If people are going after internado there might be a program to do your servicio portion there OR spain might accept 5 years unlike the u.s.. Either way the only people i saw go to spain...a) went for rotations in their internado for a couple of months or b) went after they did servicio social.
This doesn't mean they couldnt' have gone right after internado but i didnt see it.