i never found SU type carbs to have that snap it open acceleration that even cheap CV bike carbs have. A carb for each cylinder would help and cheap to setup.

Peter Zekert has cams galore ready to go almost any grind. As DD stated that cam is the torque and low rpm cam you best use a GX cam it brings the starting sweet spot much earlier in the 2800rpm range in an a15 well it did for my A16 as compared to the usual 4000rpm as it does in the A12gx engine.

What ignition and flywheel do you run?

any pics of the car? I know its not a datsun but Im a sprite and midget perv and yours has a datsun heart in it so qualifies somewhat here.