Well, this is a big subject but I will do my best to try and help.
In short, your best bet would be to replace your existing box with something stronger because it is a very time consuming and expensive exercise to strenghten your current box. You can expect to pay around $1000 for a straight cut gearset and idlers, then extra for any other necessary mods to make it all work and handle the power output of your engine.There is a fair amount of machining and TIG welding required to get it all to work.
I know you want to keep things "datsun" but replacing your existing box with something else "bigger datsun/nissan" would be by far the cheapest and simplest option, especially if it has to cope with 300 Hp.
Normal gearsets inside your gearbox are helical, that means when they are cut they are cut on a sloping angle to the gear shaft axis. The gears when in operation sort of slide over each other and the loads are distributed over a smaller area than a straight cut gear ie:not as strong, they also apply more side load to the operating shaft. Straight cut gears are just that, gears that are cut straight in relationship to their axis. they dont have the gear teeth engage on a sliding angle. The load that a straight cut gear can handle is far greater than a helical gear because the surface area to which the load is applied is greater therefore distributing the load over a larger tooth area, they also apply less side load to the operating shaft.
I tend to wear syncro's at a far greater rate and the steel ones are getting hard to find - Usually ex works stuff.
If you still want to modify your box after everyone's had their say on the subject and you want it done properly then you should budget at least $2000 - $2500. What you could end up with is a strong gearbox with a straight cut gearset, stronger/larger front shaft, Alluminuim oxide bushes and thrust washers and quick shift with modified detents.
It's an aweful lot of work and a lot of developement time, that's why I suggest you look at other options for a streetcar.
Hope this helps