Brum,
These carbs have got a bad reputation, "boat anchors" phrase attached to them. Mainly as they are more difficult to tune, but if they are understood, they can be better than the Hitachi HJG46W 240z carb. eE.g, the flat top carb, HMB46W has a power valve, which puts more fuel in at a certain high rpm. These carbs like others, only have three items to meter, fuel, air and vaccum, how hard is it?
Not many fans of these carbs here (I am one), there is a thread on classiczcars.com which has some info. There is good info in the Nissan workshop sevice manual covering the L20A to L28 engine, green cover. There was a few Nissan sevice up-dates for the North American market, as they had "problems". Getting and keeping these to factory adjustment is key. Spares, very hard to come by, sometimes see a service kit on Fleabay, I have a few bits.
I would say the HMB airbox is better than the HJG one, as Nissan changed the design, which to me makes sense.
Get the manual/understand the carbs first, I will help where I can.