V-A-C-U-U-M,... Vacuum

The E49 Valiant chargers from the 70's didn't use power brake boosters. Why?... With triple Webbers, they just didn't develop any significant vacuum signal, so the booster was just extra weight to cary.
This engine used over 88 cubic inches of swept volume to draw air from one Webber while even an A15 uses only about 46 cubic inches.

The Datsuns auto trans was designed to work with the vacuum signal that is generated by a standard engine. If one fits dual Webbers [or similar] the vacuum signal is significantly reduced, throwing all the shift characteristics out the window, which is what is being experienced here. Hooking the signal from the other two cylinders into the line should help, but probably not by much. A hot cam will just about destroy any vacuum signal completely.

Talk to a savvy trans specialist & rebuild the box with stronger components where available, see if there is a fix for the vacuum modulator problem & this may mean a full manual over-ride, & use a higher stall torque covverter to allow the engine to devlop a little torque before you launch.

I suspect that about $1000 should go a long way towards making it all work.