Taking it apart and cleaning the cylinders is definitely the best idea, but if you just aren't willing to do that, there is an alternate way. I know an old timer who has one of those garages with about 50 engines laying around and he's run into the same problem as you when he goes to use one of them in something. He puts it on an engine stand and points one row of cylinders towards the floor and soaks the rust with wd-40, then uses a green pot scrubber to carefully rub the rust off. Then he rinses out the cylinders with more wd-40. The trick is NOT to rotate the crank before or while you do this... just put the engine on the stand like it sits right now and do what I explained. Clean as much of each cylinder as you can on one bank, then do the other bank and THEN slowly rotate the crank... some pistons will move up and some will move down. In the cylinders that the piston moves lower in the block, repeat the process to finish cleaning what you couldn't get to with the piston in it's original position. Then rotate the crank in the opposite direction as before and clean the rest in the same fashion. This will minimize damage to the rings and help make sure you got all the debris out of the cylinders. When you rotate the crank, only do it enough to make the pistons move about one inch in each direction... you don't want to drag the rings through rust any further than you absolutely need to. I hope this idea helps you and you should be fine even though it's not the BEST way to clean rust from you cylinders.