Here is the MC-9 manual section on suspension. The air bag dimensions are shown for both convoluted and rolling lobe air bags. http://www.buses101.com/PDF_Files/MCI%209%20Maint%20Manual%20in%20PDF%20format/MC-9%20-%20Maint%20Manual%20Section%2012.pdf
Putting block-off plates on convoluted springs will indeed make it ride a lot stiffer, since you lose the majority of the air volume in the spring system by not using the air beams. Fact of life. Don't forget that the front suspension won't be equal unless the bus happens to be level at the rear, so you could measure both sides and take the average. Once you do it the first time, you can take external measurements for future reference.