You will probably need a different cross member and engine mounts to begin with, if you are keeping the 4 speed box then you should be able to bolt it straight on and not have to modify the tunnel at all, if you are changing to the 5 speed unit then you will need to modify the tunnel with either a big hammer or with a welder.

The 5 speed box is a bit longer so will need the hole for the gear stick moved and also the gearbox mount will need to be moved back.

As for the wiring, new starter motor lead will be required as the starter is on the other side, you may want to take the time and move the battery to the boot for extra room, all the other stuff, carb, dizzy, oil and water wires can be extended provided the dizzy is running points, if not then a decent electronic ignition system would be advisable.

Try www.speedshack.co.uk for all the bits you need, they do mail order.

Hope this helps