Valve seal replacement

My 2 cents worth. I would use the rope. Why? Because the 198 and 225 slant six engines have the piston down in the bore at top dead center. If using air, you need to make sure the piston is exactly at TDC or the air pressure will push the piston down. Also if you break the air pressure seal (like when removing or installing the keepers) the valve can drop to a point where you cannot grab it to pull it up (even if the piston is at TDC). Don't have that problem using the rope.
EDIT: Depending on your budget and ambition, and what you want from the car.
I would consider removing the head and having the guides checked (and repaired if needed) freshening up the valve job, and milling the head to get a HONEST (by measuring) 8.2-8.5 compression ratio.