Another thing to keep in mind is that its a good oppertunity to put in a completely different engine and gearbox combo rather than just the gearbox.
Also, simply stepping up to 215x75R15s from the stock tire will help raise the ratios a bit and dont need any modification (except maybe the front outriggers), and look and feel much better on the road.
I used to do thousands of Ks in my SWB, and always used to wish for the 5th gear. after about 15,000kms of highway in a 6 week period of me sitting on >100km/h for hours everyday the motor siezed on startup, I used the money from the contract to buy a newer car and gave the sierra away
It only had one straight panel (the bonnet) from a really good roll down a sand dune (3 rolls and a flip) and a dead engine, but it was still the worst car decision I ever made.. That is untill I practicallly gave away my ( $2k Heavily modified

) sierra that also had a dead engine and only 2 straight panels..(I had a spare engine, but was moving over here) and got a jeep. but I digress.
Anyway, my 2c is if your going to keep it stock, keep it stock.
If not do it properly and put in some power, people that do want to keep it stock will buy the 1l +gearbox (they are cheap bastards) to help offset the cost.