It all depends on what you will actually be using the car for, & how much you wan't to spend, but if you want to be at least a little bit competitive, then your friendly cam grinder will be the best place for good advice there.
If you can come up with an affordable GX head & carbs, then that will be a good start. Failing that, try looking at A14 heads. These had bigger ports & valves than A12's & the GX type oval ports & manifold bolt pattern. Make sure that the head that you look at does not have a noticeably bigger combustion chamber than an A12.
If you use the A14 head, simply overhaul it, & i'm sure that many here can tell you how much to shave it to restore compression. You would also need to check to see if someone has a "pre loved" [used] manifold & carb for sale. A pair of SU's or a stock GX setup in good condition should be satisfactory, but a single side draught will also be good.

A GX exhaust manifold is also good, but a set of extractors may well be easier to find.

There you have it. Relatively cheap & cheerfull, but it should give a good performance. Add electronic ignition too if you can.

One of my friends ran a Holden 202 engine on ONE throat of a Webber carb on his panel van both on the street & on the show circuit back in the late 70's / early 80's. This thing was turbo'd with 9lb's of boost & ONE throat was all it needed to light up the tyres without problems.
For those old enough to remember, it was a green HQ called "Midsummer Dream"

I'm sure that there will be others who will have their own recomendations, but Feral will be a good one to take heed of.
Chris
P.S.
Bullit
Rejetting will NOT cure the symptoms of OVERCARBURETION. This is a situation where the gas velocity at wide open throttle drops below the critical point where it can not draw fuel through the fuel delivery nozzle.

By way of illustration, imagine a 4bbl carb on a lawnmower engine. Now thats GROSS overcarburation & you would be lucky to get it to run at all, but HEY, it's a big carb, so it should mow the yard it 100mph.

In Webbers, the usual fix is to fit smaller venturi's, which in effect is re-sizing the carb. For the well heeled, this is a good way to go as this method, at great cost, effectively converts two carbs to equal one standard Webber carb, while allowing scope to go bigger when the need or situation arrises, but my suggestion is for a more budget oriented engine.

Anyway, what does everyone else think?