Xj 550 but fj 600 motor question

hello from Mexico guys this is my first post,
well to be quick, i bought a yamaha Xj 550 seca to set up a cafe racer with the motorcycle but i found something interesting,
i am looking for help about what kind of motor i have because i found a 600 cc engine on the bike, the bike frame and the engine has a VinNumber like 4u8- something but the cylinders has a 600cc, so i thought the last owner had changed the engine

but when i proceed to buy some new parts to restore the engine i found some crankcases from the FJ 600 with vin number 49A-something

the question here is, the 4u8 comes from a crankcase of xJ550 ant the last owner changed the Cylinder
or if the 4u8 vin comes too for the FJ 600 models and i have a FJ600 engine on a xJ 550 frame

 

last owner changed the cylinders we have someone here who done this too

4U8 is 81 through 83 550

 

thank you so much thats interesting, i got another question, the crankshaft and the connecting rod are the same on xj 550 and the fj 600? this is because, haha i bought today a the cranksahft and the connecting rods from a fj 600 and i wanna know if the pieces fit on the xj 550 cranckcase

 

550 has a shorter stroke than the 600.

 

550 crank shaft is different part number than fj600

 

I'd take a guess and I think with the right bearings it would fit the cases, pretty sure the 600 rods are shorter than the 550 but with the longer stroke work with the block ok