Since weve been searching for a 5 speed for A series I thought since Ive been through quite a few yards in my 28 years as a grease junky and have to say the best racing ratios in a small sized diff commonly available are from Suzuki sierra front and rear and Vitara front diffs which are been wrecked in larger numbers the last 2 years and are closest to the H165 that peeps use often.
The vitara rear is much larger again so the front diff R&P is preferred not the alloy casing unless it can be modded to feed oil to the front bearing which is doesnt so best use the sierra cast iron carrier which is small anyways.

here are pics of the diffs and the front often have a locker supposedly
http://www.zukikrawlers.com/showthread.php?t=4215

here are the ratios...

Years....2 or 4 door...Engine type...Transmission...Axle ratio
89 2 1.3L carb. 5 speed man. 5.625
89-95 2 1.6L TBI 5 speed man. 5.125
89-95 2 1.6L TBI 3 speed auto 4.625
91 4 1.6L TBI 5 speed man. 5.375
91 4 1.6L TBI 3 speed auto 4.625
92-98 4 1.6L MPFI 5 speed man. 5.125
92-98 4 1.6L MPFI 4 speed auto 5.125
94-98 2 1.6L MPFI 5 speed man. 5.125
94-98 2 1.6L MPFI 3 speed auto 4.30
94-98 2 1.6L MPFI 4 speed auto 5.125
96-98 X90 1.6L MPFI auto or man. 4.625
96-98 4 door 1.8l 16V DOHC MPFI manual or auto 5.125

X-90 4.875 gears have an 8/39 tooth count


4.30 gears have a 10/43 tooth count
4.625 gears have an 8/37 tooth count
5.125 gears have an 8/41 tooth count
5.375 gears have an 8/43 tooth count
5.625 gears have an 8/45 tooth count

datsun h165 c20 vans are rare these days so hope this helps.