2" is too small for anything other than a standard A15. 2 1/4" is fine for good midrange power but for a full out 8000rpm A15 2 1/2 is best.
We actually used 2 1/2 on our best 1300 Rally motor because it was working between 5500 - 8500rpm.
Regarding the carbs, I would tend to stick with 40mm, unless again it is a full house 8000rpm monster. It will definately help the torque with the smaller carbs.
In regard to Dynos in the NI I can't actually recommend any in particular, Stirlingmac may be able to help there. My only comment would be to try to find someone that uses a "sniffer" type exhaust gas analyser to check fuel mixture rather than O2 sensors.
Hope this helps.

hmmm thanks everyone. no real consensus on what's best though! I had thought 2.25" exhaust. Kelford recommended 45s.
We'll see how it goes. A datsun book I have for the L-series recommends 3" exhaust for 4 cyl L series race cars. That made me think bigger might be a better bet. We'll see. It'll probably end up with whatever is easiest to get the car running again. I'm sick of having it parked doing nothing.

from my experience, 2 inch is good but for a hi po engine, 2.25 non mandrel with one muffler and a hot dog would be good, no droning and good flow (as good as 2 inch mandrel and about half the cost if the shop does it for you).