You will, of course, need to swap your TR3 gearshift lever over to the TR4 gearbox.
In case you do not know, there are a couple of additions that some (me) feel are a good idea here. Adding a diode to make life easier for the relay and adding a fuse (I used a slo-blo) to protect the wiring should the the solenoid pull-in circuit fail to release. Several threads here on both - credit for both ideas goes to our late friend Randall.