Five Speed conversion SIIa V8

After not getting the hang of having no syncro on 1st and 2nd and I can see I am going to reck a perfetly good rebuilt gearbox, how easy is it to fit a five speed manual gearbox from a RR to my 1962 IIa it has a V8 fitted with a series IIa 4 speed and O/D behind it.Will I need new prop shafts and is there any need for diffrent gearbox mounts (i.e weilding I can weld but hate doing it).
Also I bet a RR gearbox will be quieter!!!