The A-type carries full hydraulic pressure all the time, which gets dumped more or less instantly to the pistons when the solenoid pulls in. The J-type does not carry full pressure all the time, so after the solenoid moves, the little one-lung hydraulic pump has to build pressure to operate the pistons. And since it gets only one stroke per driveshaft revolution, it can take quite some time to build pressure at 2nd gear speeds. The OD clutch will slip from the time the pressure overcomes the return springs, until it is high enough to lock the OD clutch; which could lead to rapid wear if you ever shift under power (either intentionally or accidentally).
IMO it would handle the torque OK (of a relatively stock TR6 motor), once full pressure was built up. If not, it's a fairly easy matter to shim up the pressure without removing the unit from the car. But you would need to be careful to always wait for it to build pressure before applying throttle, which to my mind would more or less defeat the main advantage of having OD in 2nd.
If your cover is not drilled for a reverse switch, then you'll have to add one. Check the reverse fork, but I believe most if not all of them had the ramp on the back to operate the switch. Check the hole location carefully, make sure it is centered over the shaft and the correct position fore/aft so the ramp will pass the switch as you shift from neutral to reverse. Then it's a simple matter of drilling and tapping to the rather unique thread involved (might be M16x1, but I don't recall for certain & don't have my materials handy to check).