Whether it's 40 or 45s (you can run _appropriate_ sized venturis in either to get the result you want) I definitely second the dcoe9 advice. I'm mostly familiar with the 45s (and using them on other cars) but basically these are the (relatively) earlier more racing oriented pieces. There are more recently manufactured ones including (iirc) 13s and 152s (or similar numbers to that) but aside from being more recent manufacture, they have differences in the idle and off idle (not sure what weber refers to these circuits as, they might use 'transition' circuit, I don't know). circuits and they tend NOT to be as tuneable with regard to optimal throttle response and part throttle smoothness. They might work well on streeters and (potentially, though I haven't seen this fully explored up close so I can't give any concrete feedback) - but for racing the dcoe 9 is the pick of the litter. If you notice webers coming up for sale here, it's actually one of the main questions potential buyers ask. There is probably a few hundred dollars in resale price between them.

THere's also some others - that have slightly different lettering to dcoe. I can't think of the letters off the top of my head, maybe dcom, but there's a couple of people selling them from time to time on ebay and their write up claims that they are just a 'modern dcoe' . This is not in fact the case, there are differences, irc the accelerator pump is a diaphragm not a plunger (and in this case is a little trickier to tune, though on other carbs the diaphragm type is usually better) and I'd put them in the same category as the later made 152s in terms of desirability.

Make no mistake, if there was nothing else available, I'd go the other dcoe variations for sure. But the dcoe 9s aren't that scarce, you _will_ be able to get them if you keep a close eye on ebay, and the various datsun (and other car) for sale forums.

A quick note on venturi sizing (and tuning in general) although maximum power is important, the car will actually be faster (and considerably so) if it has more 'average' power over a wide enough rpm range (basically whatever rpm range it has to operate in during the race). About the only exceptions are tractor pulling, land speed runs - both of which require peak power and (relatively speaking) a concentration of it up high in the rpm range (whatever rpm the engine can safely spin of course). To a lesser extent (provided it runs a really loose convertor or particularly close ratio gears and low diff gears) it is the case in drag racing.

Point being, there probably is a choke size (what weber venturi sleeves are called) that will make perhaps 2-3% more peak power, but the size just one step lower than that, although down on peak power, will likely have as much as 5-10% more power at all rpms lower than that peak hp rpm point. And such a combination will absolutely slaughter the opposition if the opposition ran the larger choke size.

So basically bigger isn't better. That said, there is such a thing as too small a venturi. It's about the 'right' size not the biggest possible.

you could actually go even further with this. twin webers (or any one carb throat per intake port setup) will not actually make the best peak power (no matter what choke sizes you run). Technically a large single carb, and large plenum will make more power. In the case of the a-series, no such manifold exists, but if it did, and was well designed and fine tnued, it would make more power. this is the case with other engines too. However webers (or individual throttle bodies) are used on practically any racing engine where they are legal to fit. Because they produce a wider powerband, and more average power (with that small peak hp drop).

The next big thing is throttle response - and properly setup webers are sensational on that front too :) Apart from accelerator pump circuit tuning and idle off idle/transition circuits (all of which improve throttle response) well apart from that, optimally sized chokes again come to the party - avoiding going 'too big' there means they develop signal strength more effectively, so they get the main fuel circuits flowing in a more responsive manner than chokes that generate weaker signals. So it's just one more benefit of not going too big on the choke sizing.