Its not hp that kills boxes but torque.

Datsun boxes are numbered by the distance between the centres of the countershaft and the main shaft. 56 series 56 mm, 60 series 60 mm, 63 series 63mm, 71 series 71 mm.

The larger the distance between shafts the larger the shaft diameters and gear sizes that can be used. With helical spiral gears, rotation of the gears under load tends to make the gears try and spread away from the other gear. The shaft must be strong enough to with stand this load without distortion.

A smaller shaft will distort more for a given load than a larger shaft.

My tip is that the A15 will produce about 150 - 160 ft/lb torque and about 230-240 hp @ 14 psi boost.

As a guide the maximum continuous torque limits of the various boxes can be determined by the ratings of the bearings or the torque the smallest diameter of countershaft or mainshaft will take without approaching plastic limits

Approximate guides are 50 % up on the stock engine torque : -

56A 4 speeds 105 ft/lbs
56A option boxes 120 ft/lb
60 series boxes 135 ft/lb
63 series boxes 175 ft/lb
71B series boxes 240 ft/lb
71C series boxes 300 ft/lb