Going 5 speed from 4 speed is not recommended. For that i have the following to say.
H 55 F have both the versions for diesels and petrol vehicles. Gears are the same, the only difference is that of the housing and the smaller shaft which comes out of the gear and fixes in the clutch plate, sorry forgetting it's name. Out of 10 H 55 F gears in pak, you'll find 9 of diesels and to make it fit into a petrol engine, you'll have to cut and weld that shaft which is not at all recommended.
A 40 is not for more than 100km even of highways. And if at all you need the rpm to be calm even on more than 100 or 110, increase the existing tire size. By the way, what tire size r u running currently. With this the rpm will decrease and it's just like changing diffs and you'll end up having more road clearance also.
And than, the original 82 FJ 40 was with the stock 3 speed petrol gear. The 4 speed only came in 83 and 84 models. So to me a 5 speed will keep you busy in changing just gears from 0 even till 100km/ ltr. The petrol H 55 F only came in an FJ 62 along with the 4 speed gear and never in a petrol 40.
And lastly what a 4X4 has to do with the 5th gear??????? Again i would say, just change the tires and get the same result.
If you need any further assistance, you can contact me on 0333 5119925. I have used all 2b, 3B, new 3B, 2H, 12HT, 13BT, 2F and the 3F with both 4 speed and 5 speed gears.