RC306 is final generation of RC303, which I know a lot about.
RC303 was made for 130mm travel. 140 won't be a problem. AFAIK, RC306 is further lightened and with post-mount rear brake fitting.
It's a very comfortable, light, do-it all frame that handles well and consensus on Retrobike underrates them IMHO. Certainly the most comfortable hard-tail I've ever had - which is where it falls down - but if you are on too tight a budget for full-suspension, it fills the gap quite well.
Twinned up with the right fork (i.e. one that will bob down when locked out when climbing) it climbs really well and weight is OK. Mine came in at 11kg with UST rims and 2.1 tyres, 180 rotors and no consideration taken with regard to weight whatsoever.
Fitting rear brake is a hassle due to the gusset at the seattube/top-tube joint. Seatpost minimum insert is pretty deep which means you might need to cut your seatpost to drop the seat far enough for extreme descents.
After all that - My size medium is for sale as F/F combo for £850, including WCS headset,Thomson seatpost and BNIB DTXCM130s.Why? So I can run a 405 instead - which is the same front end, with rear suspension!
Completed: 94 Stumpjumper w. M733; ; 92 Dave Yates 731 ; 97 Parkpre Pro-Elite ; Pace RC200 F6 ; Pace RC300; 82 Gios Super Record ; 97-8 De Rosa Giro D´Italia; ; Banani SPX Track bike ; 89 Alan/Mavic Cross Bike ; 05 Focus Cross Team ; 90 Look MI-80 all-Mavic ; 12 Focus Izalco Team/custom build ; 11 Focus Thunder Custom build
Projects : , Brouwer all-Mavic
Stripped : Pace RC303
Gone : GT Karakoram
Stayed in the family: Kuota Kharma, 04 Stumpjumper FSR
Ceased to be : Dynatech MT3, killed by corroded seatstays.