The short ones go on the exhaust. Ford used valve rotators on the exhaust to turn the valve when it seats, so that the exhaust valve and seat do not wear in one spot. Apparently it makes the valves last longer.
It is the thick style retainer, and it uses up about .25 of valve stem length.
thanks for the info.... another way to look at them is the exhaust spring coil "ends" donot overlap, whereas the intake spring ends do about 1/2-3/4 of an inch.