84 back used rear tub support bars spanning from the second to last set of body mounts to a rear floor channel. The last 2 years of the 7 did away with these and used a triangular support piece that was welded in this corner of the frame. It strengthened it and gave the body mount there double the thickness of steel to mount to.
For swapping this is not a problem other than the pre 85 folks will not be able to mount the support bars to the 85 and 86 frames and the 85/86 folks will see the frame is different in this area. With the OP in Italy, any CJ 7 frame in good shape will work. As mentioned above, the pre 82 will have different front shock towers (lower and fixed) vs the taller and bolt on. The earlier frames CAN be converted to the later shock towers but require cutting, drilling, and welding. Ive converted at least three and the trick is adding the crush tubes inside the frame.
While these little difference can easily be overcome by a seasoned Jeep guy or a fabricator, they may slow down or stop someone wanting to use their existing correct year parts on the non correct year frame. Examples include: second set of holes for the 20 gallon tank, Shock mounts on the widetrac axle vs the spring plate, rear tub support bars, inner fender clearance on the shock towers, etc, etc.
So when I suggest a certain year range, its taking all of this into consideration and making it easy for them. BUT, being in Italy, any rust free CJ 7 frame will have to work....
1975 CJ-6 1983 CJ-8
1986 CJ-7 Laredo 1986 TJ-7 Trail Jeep
2003 Inca Gold TJ Rubicon