so in 4th, the overall ratio of the 1600 is 4.41. The 1800 is 4.27 and the 2lt is 4.02.
The 1600 and 1800 are both 002 gearboxes, and can have the internals swapped. A good box is the 1800 with the 1600's 0.82 4th in it. This decreases it's overall ratio in 4th to 3.98 which is slightly better than the 6-rib.
My albins gearset cost me $350 which I think is pretty good. Then you have to get it put in as well...
Dakdat, I think your cheapest solution is to put a 3-rib 0.82 4th in your 5-rib box. This will give you better highway gearing than the 6-rib, for little cost. If you want better highway gearing you will need a custom gearset like me.
59bus, you could also go to the 5-rib with a 0.82 4th. The 5 rib will be a direct swap for your 3 rib box (may have to change input shafts depending on what your running).
Baja Wes 路
for anyone with Excel 2000, you can download a cool program I made that lets you do a heap of things (calculate acceleration), one of those things being plot gear ratio charts and calculate engine rpm at any speed in any gear.
If you think you will break a gearbox from too much HP, go for the 6-rib. It is a fair bit stronger. But it is slightly longer (10-20mm). If you've got a modest 2L datsun motor then the 1800 will probably be fine.