Can i bolt reliant gearbox straight on to diff removing propshaft to shorten the overall length of the trike ?
yes it can be done, assuming both diff and engine/g-box are all "solid mounted" (hardtailed) suggest having diff fixed on finished mounts then bolt g/box and engine to it then finally mount/fix engine and g-box in position. this way all parts have no option but to be in alignment as it will become all one solid unit. if in doubt use rubber donut as VB says.