On my old gear box as well as my new one (4 speed)
The reverse always seems a little hard to get into gear when the car is still, sometimes a little rev or moving the car forward then trying again will help it get into reverse. Any idea what's going on? This happens to both my gear boxes.

What is common with the 1200 4-speed is it's hard to put it into reverse. If you put it into first, then it will go into reverse. This happens to my 1200s. Not all the time, but some times.

maybe just due to lack of syncromesh in reverse? and that they are all 35(ish) years old now.

Yes, reverse gear has no synchro, as with most cars. When they were new, no synchro.

When my 1200 was only 10 years old, it had the problem I described. Might be the same or different from what subwoffers is experiencing.

Thanks for the info. The gear box was just rebuilt, so I was hoping it was nothing wrong with the internals.

This I also think is normal. My 2004 Nissan Pulsar is doing exactly the same. What I am doing to the Datsun and the Pulsar is put it first into second gear and then reverse. Works like a charm and goes into reverse without any problems.

the other trick that tends to work, if you put your foot on the clutch and hold it and select reverse and it won't go in, make sure the stick is in neutral, and simply lift off the clutch pedal and put it back down and hold it and now it will almost certainly go in. All it needs is the laygear to be shifted a smidge and it'll go in, which is what the lift off and back on the clutch pedal ostensibly does.

If you find it makes grinding noises when you finally engage it in this manner, it is a hint that the clutch might be a smidge out of adjustment (or even the spigot bush is grabbing a little which is rare) - and it's still spinning the input shaft when the motor is running but the clutch is pressed fully.

The whole idea is kinda variation of a theme of what to do on an old crash box if the driver stuffs up and doesn't match the engine rpms right for the gearshift, they stick in neutral, foot off clutch, hold engine revs at whatever is 'right' (low for an upshift,or blipping throttle to lift the rpms for a downshift) and then engaging the clutch to select that gear.