If I recall correctly, stock transmission that came with ALH, 5th gear was 0.756, with BEW/BRM it was 0.744.
There is certainly a diminishing-returns aspect towards selecting very tall top-gear ratios. The taller you go, the more you'll end up using 4th, which cancels out hoped-for tall-top-gear savings in a hurry. Bear in mind that a few people have smashed up the splines which engage 5th gear to the output shaft. If you smash up the splines inside the gear, you can just replace it, but if it takes out the splines on the output shaft badly enough that a new gear still won't engage them properly, now you are in trouble. Number one reason for smashing up parts like that ... driving around at too low engine revs with too much torque! The lower the revs, the more the power delivery through the transmission no longer resembles a smooth flow but more closely resembles a sequence of hammer-blows from each compression and power stroke. Lugging = BAD.
I had 0.681 in my Mk5 (BRM) Jetta and I towed a trailer with that car. But ... No 5th gear under load below 70-ish km/h on level ground with no trailer, or 90 km/h with trailer, or 100 km/h under load (uphill or accelerating). Coasting-down below that is fine. I also kept the revs above the visible-black-smoke threshold when towing (visible black smoke means gear down).
Personally, I would not want taller than 0.681 due to huge gap between 4 and 5, and certainly not with towing due to not being able to pull the taller gear. With a 6-speed conversion it would be a different matter.
If you spend most of your time above 100 km/h on motorways without major hills, I'd say go ahead and do it (0.681 or perhaps 0.658). No taller than 0.681 if you do any towing. If there are hills involved, 0.681 will keep you happier. If you drive slower than 100 km/h most of the time e.g. on secondary roads then stay with the stock ratio especially if there are hills involved, you will lug too much in 5th so you will have to shift between 4 and 5 a lot more (which wipes out your hoped-for economy advantage and is a nuisance). If you do mostly city and suburban driving, forget it.
