This is my personal experience with this issue. Modern K-66 cushion measures the closest to the original monarch cushions but do to the fact that the geometry of the meat of the monarchs and modern day k66 cushions are different. K-55's have close to the exact geometry of the original monarchs but measure very differently when comparing to a ruler.
For example, K66's, if mounted flush to the top of a unmodified rail will put the nose of the cushion high, where as k55 will put the nose of the cushion at the right height, but you lose the 50X100 playing area on a 9 ft table. I put K55's on old brunswicks if I have to just replace cushions and the customer dont want to spend the extra money to have the rails calibrated for a perfect playing field. If I ever get a customer who wants a true playing area I will pass the rails down to a Master mechanic here in southern california (SDbilliards) to calibrate them.
One benefit of just replacing Monarchs with K-55's is you gain a deeper pocket shelf, but you sacrifice the true playing field.
Its really hard to explain the geometry of the meat of the cushion and the angle in relation to the subrail angle. If you take a slice of the monarch cushion and a slice of the k66 and k55 and stack them you will see that the K55 follows a similar profile.
This is the best way i can explain it, I read one of Glen's first posts and he explains it alot better but I cant find it now.