iirc the jb3 gearbox on a r5 is the same as the ones used on some of the mk1 clios and later models? if so yes the jc5 box should fit (early 172 models with the speedo drive is the one you will need) you will also need the part of the mount that bolts to the gearbox as the three holes on the top are further apart than the jb3 boxes.
i done the exact replacement myself on my old 1.8 16v clio, correct me if iam wrong about the above ,been a while since i tinkered about with cars.
i also used the 172 clutch fork and release bearing as it gives you a slightly softer clutch pedal just had to take a few mm of the little green block on the back of the clutch pedal.
don't know if its the same on an r5 though. someone will be along to correct me anyway.