Marv is correct.
Longer than 17" will "fit" but they won't work as well as they should.
Also, being 18".. I'd imagine that they came off another brand... The valving will be all off...In that case the stockers would be better.
Best to sell those 18" shocks and buy a good set of used Float-2's or other brand shocks.
Stock gearing is 19/41... I prefer the 19/42... The 43 may work well... and give you a soft start. The 43 bottom will prob require a longer chain.
Good luck... That is one of the best sleds polaris has ever made.
I cant figure out why they are not offering the 700 CFi-4 for the Pro RMK chassis???