IMOO you would be better off doing a 260 Remington. the mag wont need to be modifed and the 260 at least according to my books will out perfom the x55. I plan on rebarreling my Saiga in 260 remington.
with that being said there is no real reason not to be able to do a 6.5x55. the mags are going to be the big issue as well as if the mag will fit in front of the hammer. the trigger guard to clearance will be even less as well. off the top of my head I would just lengthen out a fal or m14 mag .230 as that is the differance in loaded length in my books. then the mag will have to be made to fit a Sagia some how much like conversion mags. as you know the 308 Sagia has the feed ramp built into the mag. that 1/4 inch of length will surely complicate things as far as the bolt being long enough and possable interferance of the center support.
the only up side to dong this round over the 260Rem is you have some brass, im pretty sure that 260 can be formed from 308 brass (i could be wrong). the 6.5x55 is well regarded as a very accurate round and in loaded to 308 pressures would likely out perform or at least equal the 260. How much brass do you have??