if cyl came free of base, that eliminates cyl studs, does the 550 have studs/nuts fore & aft of cam journal? how about 6 mm bolts at outside of 1 & 4 cyls. If indeed, all bolts/nuts are out, look for machined slots @ screwdriver sized between cyl & head at outer area of cyl to head. engines which have been undisturbed for decades often "lock-up" at locating pins, (probly not your' case, but localized heat helps if it is) avoid the temptation to beat/lever at fins. (trust me, it's a lose/lose approch) luck g